Single-Storey Rear Extension Cost London 2026

Single-storey rear extension costs in London, 2026

A single-storey rear extension builds out from the back of the house into the garden, usually to make a kitchen-diner or family room on the ground floor. It suits most terraced and semi-detached houses, and it is the extension type most likely to go ahead without a full planning application.

The table gives our published London figures at mid and high specification. The totals include VAT at 20% and a typical allowance for professional fees. They do not include furniture, landscaping or party wall surveyor costs.

Above high specification, architect-designed schemes with luxury finishes and composite glazing sit at our ultra-premium rate of £4,500–£6,000+/m². For rear, side return, double-storey and wrap-around figures side by side, see the house extension cost guide for London.

Typical sizes, and what a 50m² extension costs

A single-storey rear extension typically adds 20–40m². Its depth also decides the planning route: under permitted development it can project up to 3m beyond the original rear wall of a terraced or semi-detached house, or 4m on a detached house. The Larger Home Extension route allows up to 6m and 8m respectively, outside conservation areas, through a prior approval process in which adjoining neighbours are notified and can object, and the council then decides.

Our published single-storey ranges stop at 40m². A larger ground floor scheme is priced from your drawings at the rate for its specification, and two published figures give a sense of scale. A wrap-around extension, which combines a rear extension with a side return infill, costs £190,000–£260,000 at mid specification, and a double-storey extension of 40–70m² costs £170,000–£320,000 at mid to high specification.

On a Victorian or Edwardian terrace, the strip beside the back of the house is typically 1.5–2.5 metres wide. Filling it is a side return extension, with its own planning and party wall issues, covered in the Victorian side return extension cost guide.

What moves the price within the range

After size, specification moves the price most, and the gap between mid and high specification is mostly glazing, kitchen and finishes. These are the decisions that put a project at the bottom or the top of its band.

Glazing and rooflights

Standard bi-fold doors sit at one end of the scale and structural frameless glazing at the other: frameless glazing can cost 5–8 times as much per metre run. A quality fixed or opening rooflight is £2,500–£8,000 depending on size and specification. Choosing a lean-to roof rather than a flat roof with a lantern also brings the cost down.

Daylight does not have to come from a fully glazed rear wall. On the ground floor rear extension at Evelyn Road in Cockfosters, rooflights set in the flat roof light the new kitchen and dining area, and the neighbours' garden views are unaffected.

Steel and the opening into the house

The larger the opening between the extension and the existing house, the heavier the structural frame. Removing a full rear wall needs a steel frame and temporary propping, typically adding £8,000–£20,000 depending on span, and multi-bay openings need significant steelwork. The break-through into the house comes near the end of the build and typically takes 1–3 days.

Foundations, ground and drainage

Foundations are strip or raft, depending on ground conditions and the structural engineer's specification. London Clay is generally buildable, but poor drainage, a high water table or nearby trees can require deeper foundations or specialist solutions. Extending significantly below the existing floor level means deep excavation, which adds £5,000–£15,000 depending on ground conditions.

Drainage connections are part of the standard scope. At Whitehall Park in Archway, the steel-framed ground floor rear extension sits on new trench-fill foundations, with full drainage and a sewer connection below ground.

The kitchen

The build price delivers a finished shell ready for the kitchen install. The kitchen itself, meaning units, worktops and appliances, is a separate budget, typically £15,000–£60,000+, and we coordinate its delivery and installation within the build programme.

Access, roof form and living in

Tight rear access on inner-London terraces adds time and cost to deliveries and waste removal. Houses in conservation areas, and those with complex rooflines such as mansard or hip roofs, cost more to extend. Building while you live in the house takes longer and costs slightly more than working in an empty property.

Permitted development or planning permission?

Most single-storey rear extensions can be built as permitted development if they stay within these limits:

• no more than 3m beyond the rear wall of the original house on a terraced or semi-detached house, or 4m on a detached house;
• no more than 4m high, and no higher than the highest part of the existing roof;
• eaves no higher than 3m where the extension comes within 2m of a boundary, with materials similar in appearance to the existing house;
• covering no more than half the garden area of the original house.

Flats and maisonettes do not have these householder rights. In a conservation area a single-storey rear extension within these limits can still be permitted development, but side extensions and the larger home extension route are not available, and an Article 4 direction can remove the rights street by street. In boroughs with many conservation areas and Article 4 directions, such as Islington, Hackney and Camden, check your street before you design. Where a project does qualify, we recommend confirming it with a Lawful Development Certificate before work starts. The fee is £274 and the decision takes up to 8 weeks.

Deeper extensions can use the Larger Home Extension route described above, which needs prior approval rather than a full application. Anything beyond those limits needs a householder planning application, which costs £548 and adds 8–12 weeks to the programme. Most are approved: London councils approved 86.9% of rear extension applications decided between September 2025 and August 2026, and the typical (median) householder decision took 8.0 weeks from validation, according to our 2026 London planning approvals study. The planning permission guide covers conservation areas and Article 4 in more detail.

Party wall and neighbours

A rear extension usually triggers the Party Wall etc. Act 1996 only where it sits on the boundary, or where its foundations are within 3m of a neighbour's structure and go deeper than the neighbour's foundations. A new wall at or near the boundary needs a Line of Junction notice, and the excavation an Adjacent Excavation notice. Both are served at least 1 month before the proposed start.

The neighbour has 14 days to respond. If they consent in writing, no surveyor is needed. If they dissent or do not reply, surveyors agree a Party Wall Award before work starts.

The building owner usually pays all reasonable surveyor costs: typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own. These costs are not in the ranges above. We prepare and serve the notices alongside the planning and building regulations work, so they do not hold up the start. The party wall agreement cost guide has the detail.

How long a single-storey rear extension takes

Construction typically takes 12–16 weeks on site. From first meeting to handover, a single-storey extension typically takes 6–10 months, because design, planning and building regulations account for 3–6 months before work starts. Where the extension is permitted development, the pre-construction phase is much shorter: typically 6–10 weeks from instruction to site start.

1. Design and drawings. 4–8 weeks. Our in-house architectural team carries out the measured survey and takes a standard single-storey rear extension through to planning submission, typically within 4–6 weeks.
2. Planning or certificate. 8–12 weeks where planning permission is needed, or up to 8 weeks for a Lawful Development Certificate. Building regulations drawings run alongside on the Full Plans route.
3. Pre-construction. Typically 2–4 weeks. Final selections are confirmed and long-lead items such as structural steelwork, windows and roof lanterns are ordered; large roof lanterns and steel-framed bi-fold doors can have lead times of 6–12 weeks.
4. Structure. Demolition and groundworks in weeks 1–3, structural frame and walls in weeks 3–7, roof on and weathertight in weeks 6–9.
5. Fit-out. First fix in weeks 8–11, plastering and second fix in weeks 10–14, finishing and decoration in weeks 13–16.
6. Sign-off. After snagging, Building Control carries out the final inspection and issues the Completion Certificate, which we obtain for you.

We have delivered single-storey rear extensions within 14 weeks on site. Most clients remain in their home throughout. Expect 1–2 weeks of higher disruption during the structural openings, and kitchen use may be disrupted for several weeks. The extension timeline guide covers each stage.

What the price includes, and what to budget separately

A properly specified rate per m² covers every stage of the build:

• Demolition, clearance and waste disposal to a licensed facility
• Excavation and foundations to the structural engineer's specification
• Steelwork, padstones and temporary propping during the opening-up
• Brick or block cavity walls to match or complement the house
• Flat roof build-up with warm roof insulation and membrane, or a pitched roof
• Bi-fold doors, sliding doors or fixed glazing
• Insulation to current Building Regulations Part L standards
• Electrical and plumbing first and second fix, with underfloor heating where specified
• Plastering, screed, floor finish and decoration
• Making good around the extension, and drainage connections

Not included: the fitted kitchen, bathroom suites and tiling (usually quoted separately or as a variation), furniture, blinds and curtains, and landscaping. Our ranges include VAT and a typical allowance for professional fees, but not party wall surveyor costs. These are the individual fees to check against any quote:

• Architect and design: £3,000–£8,000, or 5–8% of build cost for a full design and planning service
• Structural engineer: £800–£2,000, required for all extensions
• Planning: £548 for a householder application, or £274 for a Lawful Development Certificate
• Building regulations: £1,000–£3,000 in plan check and inspection fees
• Party wall surveyor: £800–£3,500 per neighbour, where needed
• VAT: 20%; extensions are standard-rated, and the zero rate for new builds does not apply

How we price a single-storey rear extension

We do not give a ballpark figure and adjust it once work is underway. After a consultation, our in-house architectural team prepares the drawings and manages planning, or we work from your architect's drawings. With the structural engineer's input on the key elements, we issue a fully itemised quotation broken down by trade, so you know what each element costs before you sign.

The contract is the JCT Minor Works Building Contract: a fixed price for the agreed scope, a defined programme and written variation instructions, with any extra cost confirmed before the work is done. Payment is staged: a 10% deposit secures your start date, stage payments follow milestones such as foundations and first fix, and a final retention is released after snagging and completion. Our 10-year workmanship warranty starts at practical completion.

If you are comparing quotes, we will go through a competitor's quote with you and explain any differences in scope or specification. Send drawings or photos through the contact form, which takes PDF, JPG, PNG, HEIC and DWG files, or call 020 8080 8240. Thinking of building up as well? The double-storey extension cost guide sets out the numbers, and our house extensions service explains how a project runs from first drawings to sign-off.

Frequently Asked Questions

Prices shown are indicative ranges based on London market data for 2026. Actual costs depend on the existing structure, specification and programme. All figures include VAT at 20% unless otherwise stated. Your own price is fixed in a written, itemised quote.