Extension Base Cost

Estimate the realistic base cost of your UK house extension in 2026 — by size, type, location, specification, access, ground conditions, VAT and professional fees. quick results, no sign-up required.

What Is Extension Base Cost in the UK?

Extension base costis the total estimated spend for the structural build of a house extension — covering foundations, walls, roof, windows, doors, flooring, plastering, and first fix electrics and plumbing. It is the core figure used by quantity surveyors, architects, and mortgage lenders to assess project viability. In the UK in 2026, base costs range from£1,500/m² to £3,500/m²depending on location, specification level, and structural complexity.

Why Use a Base Cost Estimate First?

Understanding your extension base cost before approaching contractors prevents underbudgeting — the single most common reason UK extension projects stall mid-build. This guide provides a realistic starting range so you can assess feasibility, approach lenders, and compare contractor quotes confidently.

Size & Type

Cost per m² falls as floor area increases — a 10m² extension costs more per m² than a 40m² build due to fixed overhead costs for scaffolding, plant hire, and site setup.

Location Premium

London inner costs run 30–40% above the national average. South East commands 15–25% premium. Northern England and Wales are typically 10–20% below Midlands baseline rates.

Specification Level

Basic spec uses standard fittings and off-the-shelf materials. Premium and luxury spec uses bespoke joinery, high-end glazing, and designer finishes — often doubling the base cost per m².

Extension Base Cost — Where Does the Money Go?

Understanding how extension base cost is split across trades helps you prioritise budget and identify where savings can be made without compromising structure or compliance.

Always Include a 10–15% Contingency

UK building projects almost always encounter unforeseen costs — unexpected ground conditions, services relocation, or structural surprises behind existing walls. A 10% minimum contingency is included in this guide’s figure. For older properties (pre-1980) or complex sites, increase this to 15–20%.

Extension Base Cost Per m² — UK Regional Rates 2026

The table below shows indicative base cost per m² by region and specification level for a standard single-storey rear extension in the UK. Use it as an early-stage budgeting guide, then compare it with local contractor quotes and external benchmarks such asFMB guidance,Checkatrade cost guidesandRICS BCIS.

• Region: London (Inner); Basic /m²: £2,200; Standard /m²: £2,800; Premium /m²: £3,500+; Relative Cost: Highest
• Region: London (Outer); Basic /m²: £1,950; Standard /m²: £2,500; Premium /m²: £3,100+; Relative Cost: Very High
• Region: South East England; Basic /m²: £1,800; Standard /m²: £2,300; Premium /m²: £2,900+; Relative Cost: High
• Region: South West England; Basic /m²: £1,650; Standard /m²: £2,100; Premium /m²: £2,700+; Relative Cost: Above Average
• Region: Midlands; Basic /m²: £1,500; Standard /m²: £1,900; Premium /m²: £2,500+; Relative Cost: Average
• Region: North England; Basic /m²: £1,400; Standard /m²: £1,750; Premium /m²: £2,200+; Relative Cost: Below Average
• Region: Scotland; Basic /m²: £1,380; Standard /m²: £1,720; Premium /m²: £2,150+; Relative Cost: Lower
• Region: Wales; Basic /m²: £1,350; Standard /m²: £1,700; Premium /m²: £2,100+; Relative Cost: Lower
• Region: Northern Ireland; Basic /m²: £1,300; Standard /m²: £1,650; Premium /m²: £2,050+; Relative Cost: Lowest

UK Extension Types & Base Cost Ranges 2026

Different extension types carry very different cost profiles. The base cost per m² is only part of the picture — some extension types have significant fixed costs regardless of size.

Single-Storey Rear

Most popular UK extension type. Straightforward build on standard ground. Typical cost £20,000–£65,000 for 15–30m². Permitted development often applies under 4m depth.

Double-Storey

Adds significant space with relatively low cost per m² vs single storey. Typical cost £45,000–£120,000. Always requires full planning permission.

Wrap-Around

Combines rear and side return extensions into one L-shape. Complex groundworks. Typical cost £55,000–£140,000 for 25–50m². Planning permission usually required.

Side Return

Fills the narrow unused gap beside terraced or semi-detached homes. Tight access increases costs. Typical range £25,000–£55,000 for 8–15m².

Loft Conversion

Dormer or hip-to-gable conversions add 20–40m² without touching the garden. Typical range £35,000–£80,000. Structural engineer design always required.

Garage Conversion

Most cost-effective way to add habitable space — shell already exists. Typical cost £12,000–£30,000 including insulation, electrics, plastering, and flooring.

Best Value Extension in 2026

Garage conversions consistently deliver the best cost-per-m² of usable space added. Single-storey rear extensions offer the best balance of space, cost, and planning simplicity for most UK homeowners. Basement conversions and wrap-arounds carry the highest risk of cost overrun due to groundwork complexity.

Planning & Professional Fees — Extension Cost 2026

Professional and regulatory fees are often overlooked in early extension budgets. In 2026 these typically add10–20% on top of the base build coston complex projects and must be factored in from day one. Planning permission, Building Regulations approval, party wall matters, structural design checks, drainage checks and VAT can all change the final budget.

Key Professional Fees to Budget For

• Architect / Architectural Designer:£1,500–£8,000 depending on project size and service level
• Planning Application Fee (England 2026):£548 for a typical householder application according to Planning Portal; check the local planning authority for your exact case
• Structural Engineer:£500–£2,500 for working, drawings, and site inspections
• Building Control (Full Plans):£400–£1,200 for domestic extension — varies by local authority
• Party Wall Surveyor:£700–£2,000 per neighbour affected (mandatory where applicable)
• Topographical Survey:£500–£1,500 if required for planning submission

Party Wall Act 1996 — Don’t Ignore This

If your extension is within 3m of a neighbour’s foundation, or involves work on or near a shared wall, the Party Wall Act 1996 applies. Failure to serve notice before starting work is a civil offence and can result in injunctions stopping your build mid-project. Serve notice early — allow 2 months minimum for agreement.

Related Guides

Once your extension cost is confirmed, use ourFoundation Depth GuideandDPC Height Guideto ensure your structural design meets UK Building Regulations before work begins.

Advanced 2026 Cost Drivers: Why Extension Quotes Vary So Much

A house extension cost guide is only useful when it understands the real reasons extension quotes move up or down. In 2026, the headline cost per m² is affected by labour availability, regional demand, foundations, drainage, steelwork, insulation standards, glazing choices, access, VAT and how much design detail exists before the builder prices the job. A cheap quote may not be cheaper if it excludes VAT, party wall costs, Building Control fees, kitchen fitting, drainage work or making-good to the existing house.

The guide above now adds advanced adjustments for groundworks risk, site access, property age and project route. These items are important because a straightforward rear extension on a modern detached house with driveway access is very different from a side-return extension on a Victorian terrace where materials pass through the property, neighbours are close, drains run below the build and structural steel is needed to open the rear wall.

Groundworks, Clay Soil and Drainage

Groundworks can change a build budget quickly. Stable ground with simple strip foundations is usually more predictable. Clay soil, nearby trees, made ground, old drains, public sewers and unknown fill all increase risk. Where a public sewer is close to or under the proposed extension, a build-over agreement or drain diversion may be required. For clay or tree-influenced sites, a structural engineer may request trial pits and deeper foundations before the design is signed off.

Steelwork and Open-Plan Living

Modern extensions often remove the rear wall of the original house to create a large open-plan kitchen-dining-family space. That design can require steel beams, padstones, temporary propping, engineering checks and Building Control approval. A modest opening with a standard lintel costs much less than a full-width opening with flush ceilings, multiple steels and hidden supports.

Glazing, Roof Lanterns and Doors

Bifold doors, slim-frame sliders, roof lanterns, large rooflights and premium aluminium glazing can transform the extension, but they also affect structure, thermal performance and installation time. The difference between standard French doors and a large sliding door system can be several thousand pounds. The guide includes a bifold/sliding-door option and roof type adjustment to reflect this.

The Cheapest Quote Can Become the Most Expensive

Always compare quotes using the same scope. Check whether VAT, scaffolding, skips, drainage, structural steel, plastering, decoration, kitchen fitting, Building Control, party wall costs and contingency are included. Missing items are the main reason house extension budgets fail.

VAT, Planning Permission and Building Regulations in 2026

VAT is one of the largest budget differences between an estimate and a real invoice. For most residential extension work carried out by VAT-registered contractors, homeowners should check whether the quote is inclusive or exclusive of VAT. Some conversions and special circumstances can be treated differently, so always reviewHMRC VAT Notice 708or speak to a qualified tax adviser if the VAT position is unclear.

Planning permission is separate from Building Regulations approval. Some single-storey rear extensions in England may fall under permitted development rights if they meet the conditions, but double-storey, wrap-around, side, conservation-area and listed-building projects often need more formal checks. Use thePlanning Portal extension guidanceas a starting point, then confirm with your local planning authority.

Building Regulations approval is normally required for extensions because the work affects structure, insulation, ventilation, drainage, fire safety and energy efficiency. GOV.UK explains that Building Regulations approval is different from planning permission and that you may need both. For technical guidance, review theGOV.UK Approved Documentsand theBuilding Regulations approval guide.

VAT Check

Ask whether every quote is plus VAT or including VAT. A quote excluding VAT can look 20% cheaper but may not be cheaper in reality.

Approvals

Planning permission, prior approval and Building Regulations approval are different. Your extension may need one, more than one, or specialist consent.

Party Wall

Excavation near a neighbour’s foundations or work on a shared wall can trigger Party Wall Act notices and surveyor fees.

Do Not Start Without Checking Permissions

Starting work before planning, prior approval, Building Regulations, party wall and drainage matters are resolved can cause delays, neighbour disputes, redesign fees and enforcement risk.

How to Build a Safer Extension Budget in 2026

A safe extension budget is not just one number. It is a structured cost plan with clear inclusions, exclusions and risk allowances. Before asking a builder to quote, define the floor area, roof design, glazing type, kitchen allowance, bathroom allowance, heating system, structural openings, drainage assumptions, access constraints and finish level. The more precise the scope, the more reliable the quote.

Recommended 2026 Budget Structure

• Base build:shell, foundations, walls, roof, windows, insulation, first fix, plastering and basic finishes.
• Client choices:kitchen, appliances, sanitaryware, tiles, flooring, lighting, decoration and furniture.
• Professional fees:architect, engineer, planning, Building Control, party wall and survey costs.
• Risk allowances:drainage, groundworks, structural changes, older-property surprises and restricted access.
• VAT:confirm whether quoted figures include or exclude VAT.
• Contingency:use 10% for simple builds and 15–20% for older, complex or high-risk properties.

Questions to Ask Contractors Before You Sign

• Is VAT included in this quote?
• Are structural steels, temporary works and padstones included?
• Are drainage alterations and build-over applications included?
• Are Building Control inspections and fees included?
• Is making-good to the existing house included?
• Are kitchen, bathroom, floor finishes, painting and decorating included?
• What provisional sums are included and what happens if actual costs exceed them?
• What is the payment schedule, retention and warranty position?

Best Practice for 2026 Quotes

Get at least three itemised written quotes based on the same drawings and specification. The best quote is not always the cheapest; it is the one with the clearest scope, realistic allowances, strong references, sensible programme and fewer hidden exclusions.

2026 Specification Levels: Basic, Standard, Premium and Luxury

Specification level is one of the biggest reasons two similar extensions can receive very different quotes. A basic extension may use standard masonry, simple sockets, standard radiators, uPVC or standard aluminium doors, a simple flat roof and off-the-shelf finishes. A premium extension may include large-format glazing, roof lanterns, underfloor heating, better acoustic insulation, bespoke cabinetry, porcelain patios, designer lighting and a high-end kitchen. Both projects may have the same floor area, but the final invoice can be completely different.

When using a house extension cost guide, choose the specification level that matches your real expectations rather than the cheapest setting. If you know you want a premium kitchen, large sliding doors and a roof lantern, selecting a basic cost setting will create a budget that fails later. The best approach is to create a base-build budget first, then add room-specific allowances for kitchens, bathrooms, glazing, heating, flooring and decoration.

Basic / Budget

Best for simple utility rooms, modest living space or rental improvements. Uses standard materials, simple finishes and minimal bespoke design. It still needs to meet Building Regulations.

Standard

Most common homeowner level. Good-quality doors, practical kitchen allowance, standard roof build-up, reliable insulation and sensible finishes without luxury detailing.

Premium / Luxury

Large glazing, roof lanterns, bespoke kitchen, high-end worktops, UFH, premium flooring, designer lighting and detailed finishes. Use a higher contingency for this level.

Room-Specific Cost Allowances

A kitchen extension is not priced the same as a simple lounge extension. Kitchen units, worktops, appliances, extraction, plumbing, lighting, flooring and tiling can add thousands of pounds. Bathroom and WC areas add sanitaryware, waterproofing, waste pipes, ventilation, tiling and extra plumbing. Utility rooms add drainage and appliance points. Home offices may need extra sockets, data points, acoustic insulation and better heating control.

• Kitchen-diner extension:allow extra for units, appliances, worktops, extraction, lighting and flooring.
• Bathroom or shower room:allow extra for drainage, waterproofing, tiling, sanitaryware, ventilation and hot-water capacity.
• Utility room:allow for waste pipes, water supply, appliance spaces, cabinetry and durable flooring.
• Home office:allow for data cabling, extra sockets, acoustic upgrades, heating and lighting comfort.
• Garden room / orangery:allow for glazing performance, overheating control, roof lantern specification and shading.

SEO Tip for Budget Planning

When comparing quotes, separate the extension shell from the room fit-out. A builder may quote the structural extension only, while another quote includes a kitchen, tiling, flooring and decoration. The total cost is only comparable when the scope is the same.

House Extension Timeline and Cashflow Planning 2026

Cost is only one part of a successful extension project. Timing matters because delays can create extra rent, storage costs, temporary kitchen costs, finance charges and contractor standing time. A realistic 2026 extension programme starts before anyone digs foundations. Design decisions, measured surveys, planning checks, structural design checks, drainage searches, party wall notices, Building Control and contractor availability can all affect when work can begin.

Typical Pre-Build Timeline

• Initial design and feasibility:2–6 weeks for brief, measured survey, layout options and early cost check.
• Planning or permitted development checks:allow time for drawings, submission, validation and local authority processing where needed.
• Structural design:1–4 weeks depending on complexity, steel beams, foundations, roof structure and drainage constraints.
• Building Regulations submission:timescale depends on whether using Full Plans, Building Notice or a registered building control approver.
• Party Wall notices:allow at least two months where the Party Wall etc. Act applies and neighbours appoint surveyors.
• Tendering and contractor booking:good builders may be booked months ahead, especially in London and high-demand areas.

Typical On-Site Timeline

A small garage conversion may complete in a few weeks, while a single-storey rear extension often needs several months from groundworks to finish. Double-storey, wrap-around and basement schemes take longer because they involve more structural coordination, roof work, weatherproofing and internal making-good. Programme also depends on whether the house is occupied during the build and whether temporary kitchen or bathroom arrangements are needed.

• Project Type: Garage Conversion; Pre-Build Planning: 2–8 weeks; Typical On-Site Build: 3–8 weeks; Cashflow Note: Lower risk, fewer structural stages
• Project Type: Single-Storey Rear; Pre-Build Planning: 8–20 weeks; Typical On-Site Build: 10–18 weeks; Cashflow Note: Stage payments usually follow groundworks, shell, roof and finishes
• Project Type: Side Return; Pre-Build Planning: 10–24 weeks; Typical On-Site Build: 12–22 weeks; Cashflow Note: Allow for party wall and restricted-access costs
• Project Type: Double-Storey; Pre-Build Planning: 12–28 weeks; Typical On-Site Build: 18–30 weeks; Cashflow Note: More design time and larger payment stages
• Project Type: Basement; Pre-Build Planning: 16–36 weeks; Typical On-Site Build: 24–52+ weeks; Cashflow Note: High risk; use QS and detailed contract documents

Payment Schedule Warning

Avoid paying too much upfront. A sensible extension contract normally uses staged payments linked to completed work such as deposit, groundworks, structural shell, roof watertight stage, first fix, plastering, second fix and completion. Keep written records of variations because small changes during the build can add up quickly. If the project is large, consider a JCT homeowner contract or professional contract administration.

Always Get at Least 3 Contractor Quotes

This guide provides a budgeting estimate only. Actual costs depend on your specific site, contractor, and material choices. Always obtain a minimum of three detailed written quotes from RIBA or FMB-registered contractors before committing to any extension project in 2026.

Extension Base Cost Guide — FAQs 2026

Frequently asked questions about UK house extension costs, planning, VAT, professional fees and budgeting in 2026.

£1,800 to £3,000 per m²