Two-Storey Extension Cost in London (2026): Cost, Planning & Value
A two-storey (double-storey) extension in London costs roughly £2,750–£4,500 per m² to build in 2026 — about £110,000–£220,000 all-in for a typical 25–35 m² two-floor extension once structural work, professional fees, VAT and contingency are included. Per square metre it usually works out cheaper than a single-storey extension, because the foundations, scaffold and roof are shared across two floors. This guide breaks down the numbers by specification, the planning rules that trip people up, and what a two-storey extension actually adds in value.
How much does a two-storey extension cost in London in 2026?
A two-storey extension adds floor space on both the ground and first floor at once — typically a bigger kitchen-diner below and an extra bedroom or bathroom above. Because you are building one set of foundations, one roof and one run of scaffold for two floors of accommodation, the cost per square metre is lower than a single-storey extension, even though the total bill is higher.
| Specification | Build cost (per m², excl. VAT & fees) | What you get |
|---|---|---|
| Standard | £2,750 – £3,400 | Solid shell, blockwork or timber frame, standard windows and doors, plastered and decorated, ready for fit-out |
| High | £3,400 – £4,500 | Better glazing (bi-folds/large rooflights), underfloor heating downstairs, en-suite above, improved finishes |
| Premium | £4,500+ | Structural glazing, architectural detailing, high-end MEP, bespoke joinery and bathrooms |
Indicative 2026 London figures, exclusive of VAT and professional fees.
For a typical London two-storey extension of 25–35 m² total (across both floors), that puts the build alone at roughly £70,000–£160,000, before professional fees and VAT.
Why is a two-storey extension cheaper per m² than a single-storey one?
This surprises most homeowners. A single-storey rear extension in London runs about £2,800–£5,000/m² in 2026; a two-storey comes in slightly below that per m². The reason is that the expensive "fixed" elements are shared:
- One set of foundations carries two floors instead of one.
- One scaffold and one roof cover double the accommodation.
- The same site set-up, welfare, skip and management are spread over more square metres.
So while a two-storey extension costs more in total, you are buying the second floor at a discount — often the best value-per-pound space you can add to a London house, short of a loft conversion.
What's included — and what catches people out?
A typical build £/m² covers the shell, roof, glazing to spec, electrics, plumbing, plastering and decoration. Budget these separately, because they are what blow two-storey budgets:
- Structural steels and groundworks — a two-storey opening needs heavier steelwork and often deeper or underpinned foundations, especially next to an existing party wall.
- Party wall awards — £1,000–£2,500 per neighbour on a terrace or semi, with notice served around two months before work starts.
- Professional fees — design, structural engineer, planning and building control: typically 10–15% of build cost on a two-storey job.
- VAT at 20% — most extension work on an existing home is standard-rated.
- Kitchen, bathrooms and fit-out — usually separate from the build: a kitchen runs £8,000–£30,000+, an en-suite £4,000–£10,000.
- Contingency — allow 10%. Two-storey work disturbs more of the existing house (roof junction, first-floor structure, making good), so there is more to discover.
Do you need planning permission for a two-storey extension in London?
Often yes — two-storey extensions clear the permitted development (PD) bar far less easily than single-storey ones. The rules under the General Permitted Development Order are strict:
- Maximum 3 m beyond the original rear wall. The well-known 6 m (terrace/semi) and 8 m (detached) "larger home" allowance is for single-storey rear extensions only — it does not apply to two-storey work.
- At least 7 m from the rear boundary. This is the rule that disqualifies most London terraces and semis — once you measure 3 m of depth, short London gardens rarely leave 7 m to the back fence.
- No two-storey extension under PD on "Article 2(3) land" — conservation areas, and homes under an Article 4 direction (common across Islington, Hackney, Camden and Kensington & Chelsea) — so most inner-London period homes need full planning.
- Two-storey side extensions are never permitted development. PD side extensions are single-storey only. A two-storey side return always needs a planning application.
- Roof pitch must match the existing house as far as practicable, materials must be similar, and upper-floor side windows must be obscure-glazed and non-opening below 1.7 m.
- Flats and maisonettes have no PD rights at all.
"Original house" means the property as it stood on 1 July 1948 (or as first built, if later) — earlier extensions count against your allowance. Always confirm against your borough, and consider a Lawful Development Certificate to put PD status beyond doubt before you build. See our London planning permission guide and our planning application service.
One thing competitors get wrong: a two-storey extension must meet the current Part L building regulations (the 2021 edition, in force since June 2022) — not the Future Homes Standard. The Future Homes Standard regulations apply mainly to new-build dwellings and take effect in England from 24 March 2027, so they do not govern a 2026 extension, though they signal where insulation and low-carbon heating standards are heading.
How long does a two-storey extension take?
Design plus planning plus party wall typically takes 3–5 months before you start on site (longer than a single-storey job, because full planning is more likely). The build itself is usually 16–26 weeks. Plan for roughly 8–12 months end to end.
Worked example: a 30 m² two-storey rear extension in a London terrace (2026)
| Item | Cost |
|---|---|
| Build — 30 m² @ £3,800/m² (high standard) | £114,000 |
| Party wall awards (2 neighbours) | £2,400 |
| Professional fees (design, structural, planning, building control) ~12% | £13,600 |
| Subtotal (ex VAT) | £130,000 |
| VAT @ 20% | £26,000 |
| Contingency (10%) | £13,000 |
| Indicative all-in | ~£169,000 |
Indicative 2026 London figures. A smaller standard-spec extension comes in lower; premium or larger projects run higher.
A smaller, standard-spec 24 m² two-storey extension can come in around £110,000–£130,000 all-in; a larger or premium-spec project (40 m²+, structural glazing, two new bathrooms) can run £220,000–£260,000+. The biggest swing factors are glazing, the number of new bathrooms, and how much of the existing house the new floor disturbs.
Does a two-storey extension add value in London?
A well-designed two-storey extension is usually one of the strongest value plays on a London house, because it adds the rooms buyers pay most for — an extra bedroom and bathroom, plus a larger kitchen-diner. Turning a three-bed into a four-bed, or adding a second bathroom, typically lifts both sale price and saleability.
The key constraint is the ceiling price for your street: every road has a realistic maximum that even a beautifully finished house will not exceed by much. If you are already near it, spend lighter; if you are well below it (common where a modest terrace sits among larger or converted homes), a two-storey extension is where the strongest returns usually sit. Uplift varies by borough and finish — get a local estate agent's view on your ceiling price before you fix the spec.
How to keep a two-storey extension on budget
- Get a proper Bill of Quantities so every builder prices the exact same scope — no apples-to-oranges quotes.
- Lock the specification before you tender. Changing glazing, bathrooms or layout mid-build is where two-storey budgets run.
- Use a fixed-price contract, not day rates.
- Do not value-engineer the structure or waterproofing — that is exactly where extensions later crack and leak.
- Decide early whether a single-storey side return or a loft conversion would hit your goal for less.
Related guides
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Get a free estimateFrequently asked questions
- Is a two-storey extension cheaper than a single-storey one?
- Per m², yes — sharing the foundations, scaffold and roof across two floors lowers the rate. In total it costs more because you are adding more floor area, but the second floor is the best-value space you can add.
- Do I need planning permission for a two-storey extension in London?
- Usually. Two-storey rear extensions are capped at 3 m deep and must be 7 m from the rear boundary under permitted development, and they are not allowed in conservation areas or under Article 4 directions. Two-storey side extensions always need planning.
- Do I need party wall agreements?
- Almost always on a terrace or semi. Budget £1,000–£2,500 per neighbour and serve notice around two months before work starts.
- How long does a two-storey extension take?
- Roughly 8–12 months end to end — 3–5 months for design, planning and party wall, then 16–26 weeks on site.
- Is VAT charged on a two-storey extension?
- Most extension work on an existing home is standard-rated at 20%. Limited reliefs exist for listed buildings and homes empty for two years or more — check eligibility before assuming.
- Does it have to meet the Future Homes Standard?
- No. Extensions must meet the current Part L building regulations (2021 edition). The Future Homes Standard applies mainly to new dwellings and takes effect in England from 24 March 2027.