Double-Storey Extension Cost London 2026

Double-storey extension costs in London, 2026

A double-storey (two-storey) extension adds space on two levels at once: typically a kitchen-diner or reception room on the ground floor, and one or two bedrooms and a bathroom above, sharing one set of foundations and one roof.

The table sets our published London figures for a double-storey rear extension beside the single-storey equivalent. The totals include VAT at 20% and a typical allowance for professional fees. They do not include furniture, landscaping or party wall surveyor costs.

Specification moves the price within each band. Our guide puts mid-range extension work at £2,500–£3,200/m² and high specification at £3,200–£4,500/m², and the gap is mostly glazing, kitchen and finishes. Wrap-around and side return figures are in the house extension cost guide for London.

Why it costs more in total but less per m²

A double-storey extension has the higher total price, but it is usually the better value per square metre. You dig one set of foundations and build one roof, then get two floors of space between them, so the structural cost of each square metre gained is shared across both floors. The cost per m² is generally lower than for a single-storey extension of the same total area, and each room you add tends to cost less than it would as a standalone single-storey.

The trade-off is structural complexity. The first floor structure, integrating the staircase, additional loadbearing elements and coordinating the new roof with the existing one all add to the programme and the cost, and they show up in the itemised quote, which is broken down by trade.

The rule of thumb: if you only need more living space, a single-storey extension is the efficient choice. If you also need another bedroom, a double-storey extension almost always makes better financial sense than two separate projects later.

Single to double storey: what changes

If you are choosing between building out and building out and up, these are the published differences:

• Cost: £80,000–£160,000 for a single-storey rear extension, against £170,000–£320,000 for a double-storey, both at mid to high specification.
• What you gain: a bigger kitchen and living space, against living space and a bedroom.
• Design: 4–8 weeks of drawings, against 6–10 weeks, because a double-storey has more rooms and more structural interactions to resolve.
• Planning: a single-storey rear extension is often permitted development; a double-storey almost always needs a householder application.
• Time on site: 12–16 weeks, against 18–24 weeks.
• First meeting to handover: typically 6–10 months, against 8–14 months.

A double-storey suits you if you need another bedroom or bathroom as well as living space, you plan to stay long term, and your plot and street can take the extra height without overshadowing neighbours.

If you already have a single-storey extension and want to build over it, the first question is structural: whether its foundations and walls can carry a first floor, which the structural engineer assesses at feasibility stage. We are happy to advise on feasibility and provide a preliminary budget before you commit to design fees. Our single vs double-storey comparison covers the decision, and the single-storey rear extension cost guide covers the ground floor option in depth.

Planning permission: why most need a full application

Double-storey rear extensions almost always need a full householder application. Permitted development does cover two-storey rear extensions, but only within tight limits:

• no more than 3 metres beyond the rear wall of the original house;
• no higher than the highest part of the existing roof;
• set back at least 7 metres from any boundary opposite the rear wall;
• a roof pitch matching the existing house as far as practicable;
• any upper-floor window in the side elevation obscure-glazed and non-opening below 1.7 metres from the floor.

The 7-metre setback is rarely achievable on a London house with a small garden, so very few double-storey extensions here fall within permitted development. A householder application costs £548 and adds 8–12 weeks to the programme. The statutory target is 8 weeks from validation, and the typical (median) London householder decision between September 2025 and August 2026 took 8.0 weeks.

Two-storey schemes are the extension type councils refuse most often. London councils approved 78.0% of two-storey extension applications decided between September 2025 and August 2026, against 86.9% for rear extensions, 88.2% for wraparound extensions and 86.6% for extension applications as a whole (our 2026 London planning approvals study). Two-storey extensions affect neighbours' light and outlook most, which is why they are refused more often.

That puts the weight on design. Our in-house architectural team prepares the drawings and manages the application, and we advise on the planning position for your address before design starts. In a conservation area, materials and design are restricted, specialist consultants are sometimes needed, and pitched roofs are often more appropriate. Most London boroughs also offer formal pre-application advice for a fee. The planning permission guide sets out the rules in full.

Structure and the party wall

A double-storey extension carries more structure than a single-storey one. Steelwork frames the openings into the existing house, the first floor needs its own structure, the staircase has to be integrated, and the new roof has to be coordinated with the existing one. The structural engineer, typically £800–£2,000, provides the calculations for the steelwork and foundations.

On a terraced or semi-detached house, check the party wall position early. A rear extension triggers the Party Wall etc. Act 1996 where it sits on the boundary or its foundations are within 3m of a neighbour's structure. Work on the party wall itself needs a Party Structure notice served at least 2 months before the start; a new wall at the boundary or an adjacent excavation needs 1 month. The neighbour has 14 days to respond. Surveyor costs, usually paid by the building owner, are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own. We prepare and serve the notices; the party wall agreement cost guide explains the process.

Two published projects show the structure involved. At Evelyn Road in Cockfosters, a Victorian semi-detached house was extended at ground and first floor: the original rear wall was removed and a steel frame installed to carry the loads from above, with Building Regulations approval secured before work began, as part of a whole-house project completed in approximately 24 weeks. At Dunstan Road in Golders Green, a 1930s semi-detached house gained a rear extension and a two-storey side extension under a JCT Minor Works Contract, with party wall agreements in place before the structural works.

How long a double-storey extension takes

Allow 18–24 weeks on site. From first meeting to handover, a double-storey extension typically takes 8–14 months:

1. Design and drawings. Typically 6–10 weeks, including the measured survey, the design and the planning drawings.
2. Planning. 8–12 weeks for the householder application. Building regulations drawings run alongside on the Full Plans route, and we serve party wall notices at the same time so they do not hold up the start.
3. Pre-construction. Typically 2–4 weeks to confirm final selections and order long-lead items such as structural steelwork and windows.
4. Build. 18–24 weeks on site, followed by snagging and the Building Control final inspection and Completion Certificate, which we obtain for you.

We have delivered complex double-storey and wrap-around projects within 22 weeks on site. In most cases you can stay in the house. Expect 1–2 weeks of higher disruption during the structural openings, when a temporary hoarding protects the interior until the new structure is weathertight. The extension timeline guide breaks down each stage.

What is included, and what to budget for

One contract covers the work from survey to sign-off:

• In-house architectural drawings and 3D visualisations
• Planning application and building regulations approval
• Structural engineering calculations
• Party wall notices prepared and served
• Groundworks and foundations
• Structural steelwork and masonry
• Roofing and waterproofing
• Electrics, plumbing and heating
• Plastering, decoration and finishing
• Project management throughout, and our 10-year workmanship warranty

Fitted kitchens, bathroom suites and tiling are usually quoted separately or as a variation, so the specification stays your choice. The kitchen itself is typically £15,000–£60,000+. Furniture, blinds, curtains and landscaping are outside the build. Our ranges include VAT and a typical allowance for professional fees, but not party wall surveyor costs. The individual fees to check against any quote:

• Architect and design: £3,000–£8,000
• Structural engineer: £800–£2,000
• Householder planning application: £548
• Building regulations: £1,000–£3,000
• Party wall surveyor: £800–£3,500 per neighbour, where needed
• VAT: 20%; extensions are standard-rated

Is a double-storey extension worth it?

Usually, where the house needs a bedroom as well as living space. A double-storey extension tends to add more value in total than a single-storey one, because the extra bedroom can move the house into a higher price bracket. In prime London postcodes, a well-designed double-storey extension can add £100,000–£200,000.

The ceiling price for your street still sets the limit on how much value any project can add, and building beyond it rarely pays back. That is worth checking before the design is fixed, not after.

How we price a double-storey extension

We do not give a ballpark figure and adjust it once work is underway. After a consultation and the structural engineer's input on the key elements, we issue a fully itemised quotation broken down by trade. 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.

If you already have an architect, we price from their drawings. Send drawings or photos through the contact form, which takes PDF, JPG, PNG, HEIC and DWG files, or call 020 8080 8240. We hold £10M public liability insurance, and our house extensions service explains how a project runs from first drawings to building control 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.