Extension Base Costs

By /

Complete UK pricing guide for house extension bases and foundations — covering strip foundations, trench fill, raft slabs, pile foundations, groundworks, oversite concrete, and all associated labour and material costs for 2026.

Jump to Section

What Is Included in Extension Base Costs?

Thebase costof a house extension covers all work from breaking ground to the finished ground floor slab — ready for the superstructure (walls, roof, windows) to be built. It is the most variable and unpredictable element of any extension budget, because ground conditions, soil type, tree proximity, drainage requirements, and access all directly affect cost. The base typically accounts for25–40% of the total extension build costfor a single-storey rear extension.

What “Base Cost” Includes — and What It Does Not

Base costs covered in this guide include:site clearance, excavation, removal of spoil, foundation concrete (all types), DPC (damp proof course) installation, oversite concrete or suspended floor slab, hardcore sub-base, drainage alterations, and concrete beam-and-block or insulated ground floor. They do not include: structural engineer’s fees, building regulations fees, planning fees, scaffolding, wall construction above DPC, or any mechanical/electrical first fix. These are covered separately in our fullExtension Cost Guide.

Groundworks & Excavation

Setting out, machine excavation of foundation trenches, removal of spoil to licensed tip, hand trimming, and any tree root or service diversion works. Typically £800–£2,500 for a 20m² extension depending on soil and access conditions.

Foundation Concrete

Ready-mix concrete placed in excavated trenches or onto prepared ground to form strip, trench-fill, raft, or pad foundations. Typically £1,200–£3,500 for a 20m² extension depending on foundation type and depth.

Oversite & Ground Floor Slab

Hardcore sub-base (MOT Type 1), sand blinding, DPM (damp proof membrane), insulation (Part L compliance), reinforcement mesh, and C25/30 concrete oversite slab. Typically £1,800–£4,000 for a 20m² extension.

Extension Base Cost Overview — UK 2026 Prices

The table and cards below provide2026 UK national average costsfor each element of an extension base, broken down by foundation type, extension size, and work element. All prices includelabour and materialsunless stated. VAT at 20% applies to most new extension work — some refurbishment work may qualify for 5% VAT relief (consult your VAT adviser).

2026 Key Extension Base Cost Benchmarks (Labour + Materials, inc. VAT)

  • Work Element: Site Clearance & Setting Out; Unit: Per m²; Low: £8; High: £18; 20m² Extension Total: £160–£360; Typical Spec: Strip topsoil 150mm; Notes: More if significant vegetation/patio
  • Work Element: Excavation + Spoil Removal; Unit: Per m³; Low: £28; High: £55; 20m² Extension Total: £600–£1,800; Typical Spec: Machine dig + tip; Notes: +40–80% if hand dig required
  • Work Element: Strip Foundation Concrete; Unit: Per lin. m; Low: £85; High: £160; 20m² Extension Total: £1,700–£3,200; Typical Spec: C20/25, 600mm deep, 600mm wide; Notes: Includes labour + ready-mix
  • Work Element: Trench Fill Concrete; Unit: Per lin. m; Low: £95; High: £180; 20m² Extension Total: £1,900–£3,600; Typical Spec: C20/25, filled to within 150mm of DPC; Notes: Faster but uses more concrete
  • Work Element: Hardcore Sub-Base (MOT Type 1); Unit: Per m²; Low: £18; High: £35; 20m² Extension Total: £360–£700; Typical Spec: 150mm compacted; Notes: Includes supply, spread, and wacker plate
  • Work Element: Oversite Concrete Slab (C25/30); Unit: Per m²; Low: £55; High: £95; 20m² Extension Total: £1,100–£1,900; Typical Spec: 100mm C25/30 + DPM + insul + A142; Notes: Part L 2021 insulation included
  • Work Element: Beam & Block Suspended Floor; Unit: Per m²; Low: £65; High: £110; 20m² Extension Total: £1,300–£2,200; Typical Spec: Prestressed beams + infill blocks; Notes: Best for problem ground / clay / trees
  • Work Element: Drainage Alterations; Unit: Per m; Low: £60; High: £120; 20m² Extension Total: £350–£1,500; Typical Spec: 110mm or 160mm UPVC drain; Notes: Highly variable — check before quoting
  • Work Element: DPC (Damp Proof Course); Unit: Per lin. m; Low: £4; High: £8; 20m² Extension Total: £80–£160; Typical Spec: 2-layer 1200g polythene DPC; Notes: Typically done by bricklayer — low cost
  • Work Element: Raft Foundation (full); Unit: Per m²; Low: £120; High: £220; 20m² Extension Total: £2,400–£4,400; Typical Spec: Reinforced slab + thickened edge beam; Notes: Clay / poor bearing capacity sites
  • Work Element: Mini Pile Foundation; Unit: Per pile; Low: £250; High: £550; 20m² Extension Total: £4,000–£9,000+; Typical Spec: CFA or driven piles + ground beam; Notes: Near trees, made ground, deep bearing

Always Get a Structural Engineer’s Specification Before Getting Quotes

Extension foundation costs are impossible to quote accurately without astructural engineer’s foundation designand aground investigation(at minimum a trial pit or window sample borehole). A structural engineer’s report for a domestic extension typically costs £400–£900 — this is money very well spent, as the wrong foundation type can add £5,000–£20,000 to base costs or lead to structural failure. Submit your structural engineer’s drawing to at least three groundworkers for competitive tenders before committing to any contractor.

Extension Foundation Types — Which Is Right for Your Site?

Selecting the correct foundation type depends onsoil bearing capacity, depth to suitable bearing stratum, proximity to trees, ground water level, existing services, and the structural loadof the proposed extension. Building control will require a foundation design from a structural engineer for all extensions in England and Wales under the Building Regulations (Part A — Structure). The four main foundation types used for UK domestic extensions are described below.

Mini Pile Foundations

Most UK Domestic Extensions Use Trench Fill — Here Is Why

Since the late 1990s,trench fillhas become the most commonly specified foundation type for single and two-storey domestic extensions in the UK. The key advantages over traditional strip foundations are:faster construction(no blockwork laying below DPC in a trench),less skill requiredfor the concrete pour,better performanceon moderately shrinkable soils (the deeper concrete mass resists seasonal movement), andlower overall programme risk. The disadvantage is higher concrete volume — but with ready-mix at £95–£130/m³, the material cost increase is usually offset by the labour saving on below-ground blockwork.

Oversite Concrete & Ground Floor Slab Costs — UK 2026

Once the foundation walls are built to DPC level, the ground floor construction begins. For UK domestic extensions in 2026, there are three main options:solid concrete oversite slab(most common),beam and block suspended floor(preferred on problem ground), andinsulated concrete formwork (ICF) raft(growing in popularity for passivhaus extensions). All must comply withBuilding Regulations Part L 2021(energy efficiency) which requires minimum 75–100mm of rigid insulation under or over the slab.

  • Floor Type: Solid Slab — Standard; Build-up (bottom to top): 150mm hardcore + blinding + DPM + 75mm PIR + A142 + 100mm C25/30; Insulation: 75mm PIR (e.g. Celotex GA4000); Cost/m² (L+M): £55–£85; 20m² Total: £1,100–£1,700; Part L 2021: Compliant; Best Application: Standard domestic extensions
  • Floor Type: Solid Slab — Enhanced; Build-up (bottom to top): 150mm hardcore + blinding + DPM + 100mm PIR + A193 + 100mm C25/30; Insulation: 100mm PIR (U-value ~0.18 W/m²K); Cost/m² (L+M): £70–£100; 20m² Total: £1,400–£2,000; Part L 2021: Enhanced; Best Application: UFH, high-spec / low-energy builds
  • Floor Type: Beam & Block; Build-up (bottom to top): Prestressed beams + dense block infill + 75mm PIR + screed; Insulation: 75mm PIR between beams; Cost/m² (L+M): £65–£110; 20m² Total: £1,300–£2,200; Part L 2021: Compliant; Best Application: Sloping sites, shrinkable clay, trees
  • Floor Type: UFH Addition (wet); Build-up (bottom to top): Pipe system cast into slab or screed; Insulation: Requires 100mm PIR min.; Cost/m² (L+M): +£35–£55; 20m² Total: +£700–£1,100; Part L 2021: Must be declared; Best Application: Open-plan extensions, kitchen/diner
  • Floor Type: Insulated Raft (ICF); Build-up (bottom to top): EPS raft former + RC slab + integral insulation; Insulation: Integral 150mm+ EPS; Cost/m² (L+M): £95–£145; 20m² Total: £1,900–£2,900; Part L 2021: Exceeds Part L; Best Application: Passivhaus, low-energy, clay sites

Part L 2021 Floor U-Value Requirement for Extensions

Under the 2021 revisions to Building Regulations Approved Document Part L (Conservation of Fuel and Power), new ground floors in extensions must achieve a maximum U-value of0.18 W/m²Kas a reasonable provision for compliance. In practice, this is achieved with a minimum of75–100mm of rigid PIR insulation(such as Celotex GA4000, Kingspan TF70, or Recticel Eurowall) under the slab. For underfloor heating systems, 100mm minimum is strongly recommended to reduce heat loss downward. Your building control officer will check the U-value working — use theU-value Guideto confirm compliance before pouring.

Groundworks Labour Rates — UK 2026

Groundworks labour costs in the UK have increased significantly since 2021 due to skilled labour shortages, fuel costs, and increased plant hire rates. The following rates reflect2026 national average day ratesfor groundworkers, plant operators, and associated equipment in the UK construction market.

  • Labour / Plant Item: Groundworker (experienced); Unit: Per day; Low Rate (2026): £200; High Rate (2026): £280; Notes: CIS self-employed rate, England avg.
  • Labour / Plant Item: Groundworker (London / SE); Unit: Per day; Low Rate (2026): £260; High Rate (2026): £360; Notes: +20–35% London premium
  • Labour / Plant Item: 1.5–3t Mini Digger (wet hire); Unit: Per day; Low Rate (2026): £220; High Rate (2026): £320; Notes: Operator + machine + fuel included
  • Labour / Plant Item: 5–8t 360 Excavator (wet hire); Unit: Per day; Low Rate (2026): £280; High Rate (2026): £420; Notes: Operator + machine + fuel included
  • Labour / Plant Item: Mini Digger — Dry Hire Only; Unit: Per day; Low Rate (2026): £100; High Rate (2026): £140; Notes: No operator — you must hold CPCS card
  • Labour / Plant Item: Muck-Away (8t tipper); Unit: Per load; Low Rate (2026): £120; High Rate (2026): £200; Notes: Inert soil to licensed tip — local rates vary
  • Labour / Plant Item: Contaminated Soil Disposal; Unit: Per tonne; Low Rate (2026): £60; High Rate (2026): £180; Notes: WAC test required — price varies with contaminant
  • Labour / Plant Item: Concrete Pump Hire; Unit: Per day; Low Rate (2026): £350; High Rate (2026): £600; Notes: Boom pump — for restricted access pours
  • Labour / Plant Item: Wacker Plate Compactor; Unit: Per day; Low Rate (2026): £45; High Rate (2026): £65; Notes: For compacting hardcore sub-base
  • Labour / Plant Item: Laser Level Hire; Unit: Per week; Low Rate (2026): £40; High Rate (2026): £80; Notes: Self-levelling rotary laser for setting out

Regional Price Variations — UK Extension Base Costs 2026

Extension base costs vary significantly by UK region — driven by labour rates, plant availability, concrete delivery costs, and local market conditions. London and the South East consistently command the highest prices, while the North of England, Wales, and Scotland offer the best value. The table below shows the indicativeregional multiplieragainst the national average for a 20m² extension base.

  • Region: Inner London; Multiplier vs. National Avg.: +35–55%; 20m² Base — Low: £9,500; 20m² Base — High: £18,000; Key Cost Drivers: Labour, parking, access, congestion charge
  • Region: Outer London / Home Counties; Multiplier vs. National Avg.: +20–35%; 20m² Base — Low: £8,000; 20m² Base — High: £14,500; Key Cost Drivers: High demand, limited groundworker availability
  • Region: South East (Kent, Sussex, Surrey); Multiplier vs. National Avg.: +15–25%; 20m² Base — Low: £7,500; 20m² Base — High: £13,500; Key Cost Drivers: London commuter premium on labour rates
  • Region: South West (Bristol, Devon, Dorset); Multiplier vs. National Avg.: +10–20%; 20m² Base — Low: £7,000; 20m² Base — High: £12,500; Key Cost Drivers: Rural access, high local demand
  • Region: East of England; Multiplier vs. National Avg.: +8–15%; 20m² Base — Low: £6,800; 20m² Base — High: £12,000; Key Cost Drivers: London overspill demand, Cambridge market
  • Region: Midlands (East & West); Multiplier vs. National Avg.: Baseline; 20m² Base — Low: £6,000; 20m² Base — High: £11,000; Key Cost Drivers: National average benchmark
  • Region: Yorkshire & Humber; Multiplier vs. National Avg.: −5–10%; 20m² Base — Low: £5,400; 20m² Base — High: £10,000; Key Cost Drivers: Good groundworker availability
  • Region: North West (Manchester, Liverpool); Multiplier vs. National Avg.: −5–12%; 20m² Base — Low: £5,200; 20m² Base — High: £9,800; Key Cost Drivers: Competitive market, lower labour rates
  • Region: North East (Newcastle, Sunderland); Multiplier vs. National Avg.: −10–15%; 20m² Base — Low: £4,800; 20m² Base — High: £9,200; Key Cost Drivers: Lowest labour rates in England
  • Region: Wales; Multiplier vs. National Avg.: −8–18%; 20m² Base — Low: £4,700; 20m² Base — High: £9,000; Key Cost Drivers: Lower labour costs, good aggregate availability
  • Region: Scotland (Central Belt); Multiplier vs. National Avg.: −5–15%; 20m² Base — Low: £5,000; 20m² Base — High: £9,500; Key Cost Drivers: Lower labour rates, competitive market
  • Region: Scotland (Highlands / Islands); Multiplier vs. National Avg.: +10–25%; 20m² Base — Low: £7,000; 20m² Base — High: £13,000; Key Cost Drivers: Remote access, plant transport costs, limited contractors

Extension Base Build Process — Step by Step

Understanding the sequence of groundworks operations helps you monitor progress and manage your contractor effectively. The following sequence applies to a standard single-storey rear extension with trench-fill foundations and a solid concrete oversite slab — the most common specification in the UK.

Structural Engineer’s Foundation Design & Ground Investigation

Commission a structural engineer and arrange a ground investigation (trial pit or borehole). Receive foundation drawings specifying depth, width, concrete class, and reinforcement. Submit to building control for approval before commencing. Cost: £400–£900 (engineer) + £300–£600 (ground investigation).

Site Setup, Strip Topsoil & Setting Out

Set up site hoarding/barriers if required. Strip and remove topsoil from the footprint area (150mm typically). Set out foundation lines using profiles and string lines or laser level to the structural engineer’s drawing. Groundworker marks out trench positions for digger driver.

Machine Excavation of Foundation Trenches

Mini digger (1.5–3t) excavates trenches to the specified depth and width. Hand trim the bottom of trenches to achieve a level, undisturbed bearing surface. Check all trenches against the structural engineer’s drawing. Remove all spoil from site. Building control officer inspects trenches before concrete is poured — allow 24–48 hours for inspection booking.

Pour Foundation Concrete (Building Control Inspection)

Order ready-mix concrete to the specified class (typically C20/25 or FND mix for trench-fill). Pour and rod/vibrate to remove air voids. For trench-fill, concrete is poured to within 150mm of ground level. Building control officer (or approved inspector) must inspect and approve before covering. Obtain inspection certificate.

Lay Below-DPC Blockwork (Strip Foundations Only)

For traditional strip foundations, the bricklayer lays 2–4 courses of dense concrete blockwork (7N/mm² or 10N/mm² blocks) from the top of the foundation concrete up to DPC level. Trench fill skips this step — saving 1–2 days of bricklayer’s time per wall line.

DPC Installation & Drainage Works

Install 1200-gauge polythene DPC (damp proof course) at 150mm above external ground level on top of the foundation wall. At this stage, any drainage diversions, new inspection chambers, or soil pipe alterations are also carried out within the excavated area before hardcore is placed. Building control may inspect drainage at this point.

Hardcore Sub-Base, Compaction & Sand Blinding

Place and compact 150mm of MOT Type 1 hardcore within the foundation walls using a wacker plate compactor. Compact in maximum 100mm layers. Apply 25–50mm of sharp sand blinding over the compacted hardcore to provide a smooth, level surface for the DPM and insulation.

DPM, Insulation, Reinforcement Mesh

Lay 1200-gauge polythene damp proof membrane (DPM) over the sand blinding, lapped and joined to the wall DPC. Place rigid PIR insulation boards (75mm or 100mm, e.g. Celotex GA4000) on top of DPM. Lay A142 reinforcement mesh on plastic spacer chairs at 40mm cover. Install any underfloor heating pipework at this stage if specified.

Pour Oversite Concrete Slab (C25/30)

Order ready-mix C25/30 concrete (RC30 or PAV1) to a minimum 100mm thickness. Pour, spread, and tamp/float to a level finish. Use a power float for a smooth surface if floor finish requires it. Protect from rain and frost for minimum 72 hours. Cure by covering with polythene sheeting for 7 days. Building control inspects slab build-up before pour — obtain sign-off.

Base Complete — Superstructure Commences

Once the oversite concrete has achieved sufficient strength (minimum 72 hours, typically 7 days before heavy loading), the base is handed over to the bricklayer to commence wall construction above DPC. The groundworks phase is complete — retain all building control inspection certificates and delivery tickets for concrete and materials for your records and future conveyancing.

Work out Your Extension Base Costs With Our Free Guide

Use our freeExtension Cost Guideto generate an accurate estimate for your specific extension size, foundation type, and location. Also use ourConcrete Guideto estimate ready-mix volumes and costs before calling your supplier.

Extension Base Costs — Frequently Asked Questions

Common questions about extension foundation costs, groundworks, and base specifications for UK homeowners and builders in 2026.

  • Site clearance + setting out:£200–£400
  • Excavation + spoil removal (approx. 12–20 loads):£800–£2,200
  • Trench-fill foundation concrete (approx. 18m perimeter):£1,700–£3,200
  • Hardcore sub-base (20m² × 150mm compacted):£380–£700
  • Oversite slab (DPM + 75mm PIR + A142 + 100mm C25/30):£1,100–£1,900
  • DPC installation:£100–£200
  • Basic drainage allowance:£400–£900

Total estimated range: £4,680–£9,500

£6,000–£7,500

Building Regulations Approved Document A (Structure)

  • Standard soils (sand, gravel, firm clay):Minimum 600mm to the underside of the foundation concrete — this is the standard minimum in England under Approved Document A Table 10
  • Shrinkable clay (most common in UK):Minimum 900mm — and up to 1,500mm near trees. NHBC Standards Chapter 4.2 provides guidance on required depth vs. tree species and distance
  • Near established trees:Depth depends on tree species, height, and distance. Oak within 10m may require foundations of 2.5–3m depth or a pile-and-beam solution
  • Sloped sites:Stepped foundations required — each step must not exceed 300mm and must not create less than 300mm of concrete depth

yes — in almost all cases

  • Standard extensions on known good ground:A structural engineer’s foundation design typically costs £400–£700 and takes 1–2 weeks. This is money very well spent — errors in foundation specification are the most common cause of costly remediation works
  • Complex sites (clay, trees, sloping, made ground):A structural engineer is absolutely essential — foundation costs on difficult sites can be 3–5× higher than standard, and an experienced engineer will specify the most cost-effective solution
  • Scotland:Building Warrant applications require full structural design checks — a structural engineer is mandatory
  • Party Wall Act:If the extension is within 3–6m of a neighbour’s building or its foundations, a Party Wall Notice must be served before excavation commences
  • Traditional strip foundation:The trench is excavated to the required depth, a relatively shallow layer of concrete is poured in the bottom (typically 200–300mm deep), and then concrete blockwork is built up from the concrete to DPC level. The trench is then backfilled around the blockwork. This uses less concrete but requires more skilled bricklayer time in the trench
  • Trench fill:The trench is excavated to the required depth and then filled with concrete almost to ground level (to within 150mm of finished ground level, with just 1–2 courses of blockwork above ground to DPC). This uses significantly more concrete but eliminates all below-ground blockwork, making it faster and simpler

Cost comparison for 18m perimeter extension (2026):

  • Strip foundation: £1,700–£3,200 (concrete + labour) + £800–£1,600 (below-DPC blockwork) = £2,500–£4,800 total
  • Trench fill: £1,900–£3,600 (concrete + labour) + minimal blockwork = £1,900–£3,600 total

Trench fill is typically cheaper overall

  • Ground conditions:A site on firm gravel at 600mm depth can cost half as much as a site on shrinkable clay requiring 1,500mm foundations. This single factor can swing costs by £2,000–£8,000 on a medium extension
  • Tree proximity:NHBC rules on tree root zones can require pile foundations instead of trench fill — adding £5,000–£12,000 to base costs
  • Access restrictions:If a large tracked excavator cannot access the site, hand digging or a mini digger with multiple cycles is required — adding 40–80% to excavation costs and 2–5 days to programme
  • Drainage diversions:If existing drainage runs cross the footprint, diversion works can add £800–£3,000 depending on complexity and depth
  • Contaminated ground:Old made ground, ash pits, or industrial contamination requires specialist disposal at £60–£180/tonne — a full foundation dig on contaminated ground can add £3,000–£10,000 in disposal costs alone
  • Rock:Encountering bedrock in a trench can require a breaker attachment (£80 per hour) adding £500–£2,000 to excavation costs and significant programme delays
  • Regional labour rates:The same groundworks package costs 35–55% more in inner London than in the North of England — purely due to labour market differences
  • Time of year:Wet winter conditions slow excavation and concrete curing, increasing programme length and cost. Frost protection for freshly poured concrete adds £150–£400 in materials and labour

always commission a ground investigation and structural engineer’s design before requesting quotes

  • Building Regulations compliance:Building control will inspect the foundations before the concrete is poured and after — the concrete must be to the specified class (C20/25 minimum), correctly placed, and compacted. A self-pour using site-mixed concrete will not have a ready-mix producer’s conformity certificate (delivery ticket) — and building control may insist on certified ready-mix concrete with a delivery docket for any structural element
  • Ready-mix is strongly recommended:For foundation concrete, ordering certified ready-mix (RC25 or FND designated mix) is the only way to guarantee the concrete meets the specification. Site-mixed concrete is difficult to control to a consistent standard for structural applications
  • The savings are modest:Labour for pouring ready-mix foundation concrete on a 20m² extension is typically only £400–£900 of the total groundworks cost — a small saving relative to the risk of non-compliance or poor placement
  • What you CAN do yourself (legally):You can act as your own project manager, source your own ready-mix, and oversee the pour — provided you use a qualified groundworker for the actual work and ensure building control inspections are carried out at the correct stages
  • Oversite slab self-pour:More homeowners successfully self-pour the oversite (ground floor) slab on smaller extensions — this is lower risk than foundation concrete. Still use certified C25/30 ready-mix and ensure the correct DPM, insulation, and mesh build-up is inspected by building control before pouring
  • Low bearing capacity soils:Made ground, soft alluvial deposits, peat, or loose fill where strip foundations would require impractical depths to reach adequate bearing
  • Highly shrinkable clay (NHBC Zone 3 or 4):Where seasonal clay movement is severe enough to cause differential settlement under strip foundations — a raft can accommodate some movement without cracking the superstructure
  • Areas of mining subsidence:In former coal mining areas (parts of Yorkshire, Nottinghamshire, South Wales, County Durham), raft foundations are routinely specified to resist ground movement
  • Sloping sites with complex step requirements:A raft can sometimes be more economical than multiple stepped strip foundation levels on a steeply sloping site
  • Filled ground with variable compressibility:Where the bearing stratum is uneven or variable — a raft bridges minor variations

Cost of raft vs. trench fill (20m² extension, 2026):

  • Trench fill (standard): £6,000–£9,500 total base cost
  • Raft foundation (instead): £8,500–£13,500 total base cost — typically £2,000–£4,000 more due to additional reinforcement and concrete volume

minimum of three written quotes

What every groundworks quote must include:

  • Foundation type, depth, width, and concrete class (as per structural engineer’s drawing)
  • Excavation method (machine or hand) and disposal of all spoil (number of loads, tip charges)
  • Concrete supply — supplier name, mix class, and volume in m³
  • Hardcore type (MOT Type 1), depth, and compaction method
  • Oversite slab specification: DPM gauge, insulation type and thickness, mesh grade, concrete class and thickness
  • Any drainage works (itemised separately)
  • Programme (number of working days for each stage)
  • Payment schedule (never pay more than 25% upfront — stage payments tied to completed work are standard)
  • Whether VAT is included or excluded (always check — most building work is 20% VAT)
  • Whether building regulations compliance and inspection liaison is included

Red flags to watch for:

  • Quotes that do not specify concrete class or volume — this allows substitution of cheaper material
  • Day-rate only quotes for groundworks — insist on a fixed price or lump sum wherever possible
  • No mention of spoil disposal costs — this is frequently used as a post-contract extra
  • Large upfront deposits (over 25%) — reputable groundworkers do not require this

Yes — in almost all cases.

  • Part A (Structure):Foundation design, wall construction, roof structure — requires structural engineer’s measurement
  • Part C (Moisture):DPC, DPM, drainage
  • Part L (Conservation of Fuel and Power):Insulation under floor slab, wall and roof U-values
  • Part H (Drainage):Any alteration to existing drainage or new drainage installations

Building Regulations application options:

  • Full Plans Application:Submit detailed drawings before work starts — building control approves the design in advance. Most common for extensions — provides certainty. Fee: £150–£500 depending on local authority
  • Building Notice:Notify building control and start work — inspections happen during construction rather than pre-approval. Riskier as non-compliance is discovered during build. Fee: £150–£400
  • Approved Inspector:Use a private approved building inspector instead of the local authority — often faster and more flexible. Fee: £300–£700
  • Excavation inspection — before concrete is poured into trenches
  • Foundation concrete inspection — after pour, checking depth and level
  • DPC inspection — at DPC level before oversite work begins
  • Oversite inspection — DPM, insulation, and mesh before slab pour

Scotland:

  • Day 1:Site clearance, strip topsoil, set out foundation lines
  • Day 1–2:Machine excavation of foundation trenches (1–2 days depending on soil and access)
  • Day 2–3:Hand trim trenches, await building control inspection of trenches (allow 24–48h for inspector availability)
  • Day 3–4:Pour trench-fill foundation concrete — allow 72 hours minimum curing before loading
  • Day 5–6:DPC blockwork (if traditional strip), DPC installation, drainage works
  • Day 6–7:Hardcore sub-base — place, spread, and compact MOT Type 1 in layers
  • Day 7–8:Sand blinding, DPM, PIR insulation, A142 mesh, await building control inspection before pour
  • Day 8–9:Pour oversite concrete slab — allow 72 hours minimum before foot traffic, 7 days before heavy loading

Total programme: 8–14 working days

  • Building control inspection delays (1–3 days each stage)
  • Wet weather preventing concrete pours (particularly problematic November–March)
  • Unexpected ground conditions discovered during excavation
  • Drainage diversion complexity
  • Concrete delivery scheduling (ready-mix plants book up — order 48–72 hours in advance)

3–5 weeks

Related UK Extension & Construction Guides

Use these free resources alongside this guide to plan, specify, and budget your extension base accurately.

Scroll to Top