Single vs Double Storey Extension London: Which Is Right for You?
Single storey is simpler, cheaper, and often avoids planning permission. Two storey gives you double the floor area for 50–70% more cost and almost always needs planning permission. Here is how to decide which is right for your London home in 2026.
TL;DR - The Quick Answer
A single storey rear extension in London costs £45,000–£165,000 inc VAT and often qualifies as Permitted Development, avoiding the need for full planning permission. A two storey extension on the same footprint costs 50–70% more but doubles your floor area and is roughly 20–30% cheaper per m². The trade-off is planning: two storey extensions require full planning permission in around 95% of London cases, adding 3–5 months before work can begin. Single storey suits open-plan living space; two storey suits added bedrooms.
The Core Trade-Off
The decision between single and two storey comes down to three things: what you need the space for, whether you can get planning permission, and how much you want to spend.
A single storey rear extension creates new ground-floor living space. A two storey extension on the same footprint adds ground-floor space and a first-floor room (usually a bedroom, bathroom, or home office) above it. Two storeys cost more, take longer, require planning permission in almost all London cases, and involve more construction complexity. But they give you significantly more floor area per metre of garden used.
The key question: do you need more ground-floor space (open-plan kitchen, dining room, playroom) or more bedroom and bathroom capacity? If the answer is bedrooms, two storey is almost always better value. If the answer is daytime living space, single storey is simpler, faster, and often cheaper than the planning and build cost difference justifies.
Cost Comparison: Single vs Two Storey (2026)
The table below compares single and two storey costs for the same footprint sizes. All construction costs include VAT at 20%.
| Footprint | Single Storey Build | Two Storey Build | Extra Cost |
|---|---|---|---|
| 3m × 4m | £38,000–£58,000 | £72,000–£110,000 | +£34k–£52k |
| 3m × 5m | £45,000–£70,000 | £90,000–£138,000 | +£45k–£68k |
| 4m × 4m | £48,000–£74,000 | £96,000–£147,000 | +£48k–£73k |
| 4m × 5m | £60,000–£92,000 | £120,000–£184,000 | +£60k–£92k |
| 5m × 5m | £75,000–£115,000 | £150,000–£230,000 | +£75k–£115k |
Source: Mayfair Studio project data • Data as of 2026
All prices include VAT. Add 25–30% for professional fees and contingency for all-in budgets. Does not include kitchen or bathroom fit-out.
Cost Per m² of Floor Area
Two storey is 20–30% cheaper per m² of total floor area because you share foundations, external walls, and the roof between two floors.
Planning Permission: A Major Difference
Planning permission is where single and two storey extensions diverge most significantly. Single storey extensions can often be built without any planning application. Two storey extensions in London almost always need full planning permission.
Single Storey
Two Storey
Planning timeline difference: a single storey extension within PD can start within 4–8 weeks (just building regulations approval needed). A two storey extension requiring full planning permission adds 3–5 months before a brick can be laid. This is the biggest practical difference for homeowners with a defined timeline.
See our detailed guides on permitted development rights in London and Prior Approval (Neighbour Consultation Scheme) for the full planning picture.
Party Wall: Both Usually Need It
Both single and two storey rear extensions typically trigger the Party Wall Act 1996 in London. Any extension that reaches a boundary, involves new excavation near a neighbour's foundations, or cuts into a shared wall requires notice to be served.
Two storey extensions add one extra trigger: raising the party wall to accommodate the upper floor. This almost always requires a party wall notice even if the single-storey version of the same project would only need an excavation notice.
Single Storey Party Wall Cost
Typically 1–2 neighbours. Triggered by boundary proximity and excavation.
Two Storey Party Wall Cost
Same neighbours but raising the party wall adds complexity and surveyor time.
What Each Extension Type Is Best For
Choose single storey if...
Choose two storey if...
Single vs Two Storey: Full Comparison
| Factor | Single Storey | Two Storey |
|---|---|---|
| Construction cost (3m×5m) | £45,000–£70,000 | £90,000–£138,000 |
| Cost per m² of floor area | £3,000–£4,600 | £2,400–£3,700 |
| Planning permission | Often not needed (PD) | Almost always required |
| Planning timeline | 0–13 weeks | 12–20 weeks |
| Build time | 4–6 months | 6–12 months |
| Garden use (same footprint) | Loses garden space | Same loss, double floor area |
| Party wall | Usually needed | Almost always needed |
| Structural complexity | Lower | Higher (floor structure, staircase) |
| Disruption during build | Less | More |
| Best use | Kitchen-diner, living space | Bedroom + bathroom above |
| Value added (typical London) | £80k–£150k | £120k–£250k |
Source: Mayfair Studio project data • Data as of 2026
Which Adds More Value in London?
Two storey extensions typically add more monetary value in London than single storey, primarily because adding a bedroom has an outsized impact on asking price. London buyers pay a significant premium for an additional bedroom.
Typical Value Added (London, 2026)
Adds living quality. Property moves from tight to spacious. Strong ROI in competitive family home markets.
Adding a bedroom changes the property's classification. A 3-bed becomes a 4-bed. In Zones 1–3, 4-bed family homes sell for £150k–£300k more than equivalent 3-bed properties.
A 3m x 5m single storey extension in London costs £45,000–£70,000 to build (inc VAT) and can add around £100,000 in property value. The same footprint as a two storey extension costs £90,000–£138,000 and can add £160,000–£200,000 in value - making two storey cheaper per m² and higher in absolute return for bedroom-constrained London properties.
The ROI calculation: a 3m × 5m single storey costs £45k–£70k to build and might add £100k in value. A two storey version costs £90k–£138k and might add £160k–£200k. Both can show strong return, but two storey often wins on absolute uplift in bedroom-constrained London properties.
See our full analysis in the extension ROI guide for London 2026.
Single Storey Now, Two Storey Later?
Some homeowners build a single storey extension now (within PD, faster and cheaper) with the foundations and structural elements designed to carry a second floor later. This is called designing for future extension and can make sense if budget is the main constraint right now.
Is Building for Future Upward Extension Worth It?
Verdict: if you know you eventually want two storeys, doing it all at once is almost always cheaper and less disruptive than two separate projects.
Frequently Asked Questions
Is a single or two storey extension cheaper?
Single storey is cheaper in absolute terms: a 3m x 5m single storey costs £45,000–£70,000 to build, while the same footprint as two storeys costs £90,000–£138,000. However, two storey is cheaper per m² of floor area (£2,400–£3,700/m²) than single storey (£3,000–£4,600/m²) because you share foundations, walls, and the roof between two floors. If you need bedrooms, two storey is better value overall.
Does a two storey extension require planning permission in London?
In London, almost always yes. While Permitted Development technically allows two storey rear extensions up to 3m deep, the requirement that the property must be at least 7m from the rear boundary eliminates this option for most London houses. Expect to need a full planning application (£258 fee, 8-week determination) for any two storey extension in London.
Does a single storey extension need planning permission?
Often not. A single storey rear extension up to 3m deep (terraced and semi-detached) or 4m deep (detached) can be built under Permitted Development with no planning application. Between 3–6m deep, Prior Approval is needed under the Neighbour Consultation Scheme. Over 6m, full planning permission is required. Conservation areas and Article 4 direction zones may remove PD rights entirely.
Which adds more value: single or two storey extension?
Two storey extensions typically add more monetary value in London because adding a bedroom has a larger impact on sale price than adding a reception room. A well-executed two storey extension can add £120k–£250k, while a single storey typically adds £80k–£150k. The value uplift from going from 3 bedrooms to 4 bedrooms is particularly large in London's family home market.
How much bigger is a two storey extension than single storey?
A two storey extension on the same footprint gives you exactly double the floor area. A 3m x 5m single storey creates 15m² of new space. The same footprint as two storeys creates 30m² (15m² ground floor plus 15m² first floor). This is why two storey is better value per m² even though the build cost is 50–70% higher.
How long does a two storey extension take compared to single storey?
A single storey extension built under Permitted Development can start within 4–8 weeks and takes 4–6 months to build. Total time from architect appointment to completion is typically 7–10 months. A two storey extension requiring full planning permission takes 3–5 months just for planning, then 6–12 months to build. Total project time is typically 14–20 months. The planning process is the biggest time difference.
Can I build a single storey extension now and add a second floor later?
Yes, but it is almost always more expensive and disruptive than doing both at once. Two separate projects mean two sets of preliminaries, scaffolding, and site setup costs. The second phase still requires planning permission and party wall notices. If you know you will want two storeys eventually, building to that structural specification from the start (stronger foundations, £3,000–£8,000 extra) saves money long-term.
Do both single and two storey extensions need a party wall agreement?
Usually yes, for both. Any extension that reaches a boundary or requires new excavation near a neighbour's foundations triggers the Party Wall Act. Two storey extensions add an additional trigger: raising the party wall to accommodate the first floor. Both typically cost £1,500–£5,000 per project for party wall surveyor fees, though two storey projects with complex party wall raising can cost more.
Summary: Which Should You Choose?
- Need a bedroom? Two storey. It adds more value than a reception room and is cheaper per m².
- Want to avoid planning? Single storey at 3m depth. Often Permitted Development with no application needed.
- Budget is tight? Single storey is 50–70% cheaper on the same footprint and can start faster.
- Maximising floor area per m² of garden? Two storey gives double the floor area from the same footprint.
- Conservation area? Check with your architect first. Two storey may be refused on scale grounds.
- Thinking of adding a second floor later? Do both at once. Two separate projects always cost more in total.