Two Storey Extension London 2026: Costs, Planning and Design
A two storey extension doubles your floor area from a single footprint. Here is everything London homeowners need to know in 2026: real costs, planning permission requirements, party wall implications, and whether two storeys genuinely beats single-storey on value.
Quick Answer
A two storey rear extension in London costs £72,000–£220,000 for construction inc VAT depending on size and specification. A 3m x 4m two-storey costs £72k–£110k; a 4m x 5m costs £120k–£185k all-in. Planning permission is always required. Party wall agreements are almost always needed. The upper floor typically adds a bedroom and bathroom.
£72k–£110k
3m × 4m two-storey
£120k–£185k
4m × 5m two-storey
Always
Planning required
Check your specific property constraints
Free Property CheckWhat Is a Two Storey Extension?
A two storey extension adds living space on both the ground and first floor from the same structural footprint. The ground floor typically becomes a new kitchen, dining room, or living space. The first floor adds one or two bedrooms, a bathroom, or a home office directly above.
Because you are building one set of foundations and one roof to serve two floors, two storey extensions are significantly more cost-efficient per square metre than building a second single-storey extension separately. The structural work is more complex than a single-storey build, but the cost per m² of usable floor space is lower.
The most common use case: a 3m or 4m two storey rear extension that creates an open-plan kitchen-diner on the ground floor and a new bedroom with en-suite above. For a growing family in a London terrace, this is often the most cost-effective way to add both communal and private space in one project.
Two Storey Extension Costs London 2026
Two storey extensions cost around 50–70% more than a single-storey extension of the same footprint, not double. You share foundations, external walls, and (in part) structural elements. The additional cost is primarily the floor structure, staircase, and first-floor fit-out.
| Footprint | Total Floor Area | Construction Cost | All-In Budget |
|---|---|---|---|
| 3m × 3m | 18m² | £54,000–£83,000 | £72,000–£105,000 |
| 3m × 4m | 24m² | £72,000–£110,000 | £95,000–£140,000 |
| 3m × 5m | 30m² | £90,000–£138,000 | £118,000–£175,000 |
| 4m × 5m | 40m² | £120,000–£184,000 | £155,000–£230,000 |
| 4m × 6m | 48m² | £144,000–£220,000 | £185,000–£275,000 |
| 5m × 5m | 50m² | £150,000–£230,000 | £195,000–£290,000 |
All construction costs include VAT at 20%. All-in budget adds architect fees (10–12%), structural engineer (£2,000–£4,000), party wall surveyors (£1,500–£7,200), planning fee (£258), and 10% contingency. Does not include kitchen or bathroom fit-out.
Full Cost Breakdown: 3m × 4m Two Storey Extension (Mid-Range)
Kitchen and bathroom costs are separate. A new fitted kitchen adds £10,000–£25,000. An en-suite bathroom adds £8,000–£18,000 depending on specification. A family bathroom adds £6,000–£15,000. Budget these separately from the extension shell cost.
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Planning Permission for Two Storey Extensions
In practice, almost all two storey rear extensions in London require full planning permission. While Permitted Development technically allows two storey rear extensions under specific conditions, those conditions are rarely met in London.
PD Rules for Two Storey Rear Extensions
Under Permitted Development (Class A), a two storey rear extension is allowed only if ALL of the following conditions are met:
The 7m rear boundary rule eliminates PD eligibility for the vast majority of London terraced and semi-detached houses. Most London gardens are 8–15m long, putting the rear boundary within 7m of where the extension would sit. Add conservation area Article 4 directions that remove PD rights entirely in many inner London boroughs, and the result is clear: plan for full planning permission.
Planning Application
Key Planning Considerations
The biggest planning risk for two storey extensions is overlooking. First-floor rear windows in an extension can look directly into neighbouring gardens and rear rooms. Planners scrutinise window placement carefully. Your architect should position windows to minimise overlooking while maximising light, typically using high-level windows, rooflights, or obscured glazing on sensitive elevations.
Party Wall Requirements
Two storey rear extensions almost always trigger the Party Wall Act 1996. The most common trigger: raising the height of the party wall to accommodate the first floor. Even where the party wall is not structurally altered, new foundations close to the boundary require excavation notices.
Party Wall Triggers for Two Storey Extensions
For a mid-terrace house, budget £3,000–£7,200 for party wall costs (two neighbours, both surveyors). Start the party wall process as soon as planning permission is granted. The statutory notice periods mean there is no shortcut: serve notice early or accept it will delay your start on site.
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Check now — it's freeTwo Storey Extension Design Options
Rear Two Storey Extension
The most common type. Extends the full rear of the house on both floors. Ground floor creates open-plan kitchen-diner. First floor adds one or two bedrooms above. Works well on terraced, semi-detached, and detached houses.
Planning note: overlooking is the primary concern. Rear first-floor windows need careful positioning.
Side Two Storey Extension
Extends the side of the house on both floors, typically filling the gap between the house and the boundary. Ground floor creates utility room, study, or extra reception space. Upper floor adds a bedroom or bathroom. Less overlooking risk than rear extensions.
Planning note: must not exceed the width of the original house or dominate the street scene. 'Wing' extensions that set back from the main roofline are more likely to be approved.
L-Shape or Wrap-Around
Combines a two storey rear extension with a single-storey side return (or vice versa). Creates maximum ground-floor space. The upper floor sits over the rear section only, while the side return remains single-storey. This is common on terraced houses where the side return is too narrow to add a full upper floor.
Cost: £140,000–£250,000 for construction inc VAT depending on size and specification.
Over-Garage or Outrigger Extension
Adding a first floor above an existing single-storey element (integral garage, original rear outrigger) without extending the footprint. Significantly cheaper than a new two storey build because much of the ground-floor structure already exists. Typical cost: £40,000–£80,000 for construction inc VAT.
Planning: usually requires full permission. Structural assessment essential to confirm the existing structure can carry the additional load.
Two Storey vs Single Storey: Which Is Better Value?
The decision between single and two storey is not just about budget. It depends on what you need, what you can get planning permission for, and how much garden you are willing to give up.
| Factor | Single Storey | Two Storey |
|---|---|---|
| Cost per m² of floor area | £3,000–£4,600 | £2,400–£3,700 |
| Planning permission | Often PD or Prior Approval | Almost always required |
| Garden impact | Loses garden space | Same garden loss, double the floor area |
| Best for | Kitchen-diner, living space | Adding bedroom + bathroom |
| Disruption during build | 4–6 months | 6–10 months |
| Value added (typical London) | £80k–£150k | £120k–£250k |
The value case for two storeys: In London, adding a bedroom typically adds more value than adding a reception room. A 3-bed house worth £700k becomes a 4-bed house worth £800k+ with a well-executed two storey extension. The cost-per-m² efficiency of two storeys over single storeys is also genuine.
See our guide on extension return on investment in London for detailed figures by extension type and borough.
Two Storey Extension Timeline
| Phase | Duration | What Happens |
|---|---|---|
| Architect design | 6–10 weeks | Feasibility, drawings, planning documents |
| Planning application | 8–13 weeks | Submission to decision (add time if appealed) |
| Party wall process | 2–4 months | Can run in parallel with planning |
| Tender and appoint builder | 4–8 weeks | Get 3 quotes, check references |
| Construction (small) | 5–7 months | 3m × 4m footprint |
| Construction (large) | 8–12 months | 4m × 5m+ footprint |
From first conversation with an architect to moving back into a completed extension: allow 14–20 months for a typical two storey rear extension in London. The party wall process can run in parallel with planning, which saves time. Start party wall notice as soon as you submit the planning application.
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Frequently Asked Questions
How much does a two storey extension cost in London in 2026?
A two storey rear extension in London costs £72,000–£220,000 for construction including VAT in 2026, depending on size. A 3m x 4m footprint (24m² total floor area) costs £72,000–£110,000 for construction and £95,000–£140,000 all-in. A 4m x 5m footprint (40m² total) costs £120,000–£184,000 to build and £155,000–£230,000 all-in. These figures do not include kitchen or bathroom fit-out.
Do I need planning permission for a two storey extension?
In London, almost always yes. While Permitted Development technically allows two storey rear extensions up to 3m deep if the property is at least 7m from the rear boundary, very few London properties meet the 7m rule. Additionally, many inner London boroughs have Article 4 directions that remove Permitted Development rights entirely. Budget for a full planning application (£258 fee, 8-week determination period).
Is a two storey extension cheaper per m² than single storey?
Yes, by about 20-30%. A single storey extension costs £3,000–£4,600/m² inc VAT. A two storey extension costs approximately £2,400–£3,700/m² of total floor area because you share foundations, external walls, and the roof between two floors. You lose more garden per m² of floor space from a single-storey extension. Over the same footprint, a two storey gives you double the floor area for about 50–70% more cost.
How long does a two storey extension take to build in London?
The construction phase takes 5–12 months depending on size. A 3m x 4m two storey takes around 5–7 months to build; a 4m x 6m takes 8–12 months. Add planning (3–5 months) and party wall process (2–4 months, often run in parallel) and the total project timeline from first architect meeting to completion is typically 14–20 months.
Do I need a party wall agreement for a two storey extension?
Almost always yes. Two storey rear extensions in London almost always trigger the Party Wall Act because they typically involve raising the party wall (to accommodate the first floor), excavating new foundations close to the boundary, and often installing RSJ steelwork in shared walls. Budget £1,500–£7,200 for party wall costs depending on how many neighbours are affected.
What goes on the upper floor of a two storey extension?
Most commonly: one bedroom (often with en-suite bathroom), a home office, or a family bathroom. In larger extensions (4m x 5m+), you can fit two bedrooms side by side on the upper floor. The most popular configuration in London is a new open-plan kitchen-diner on the ground floor with a master bedroom and en-suite above. Bathrooms and bedrooms on upper floors require compliance with Building Regulations for insulation, ventilation, and fire safety.
What is the difference between a two storey extension and a loft conversion?
A two storey extension adds entirely new space beyond the existing footprint of the house, on both ground and first floor levels. A loft conversion converts the existing roof space into habitable rooms without extending the footprint. Loft conversions are generally cheaper (£40,000–£80,000) and often do not need planning permission. Two storey extensions cost more and almost always require planning permission, but they add more total floor area and do not require structural changes to the existing roof.
How much value does a two storey extension add in London?
A well-executed two storey extension that adds a bedroom to a London property typically adds £120,000–£250,000 to the property value, depending on location and what the property sells for. In Zones 1–2, the value uplift from going from 3 to 4 bedrooms is often £150,000–£300,000. Extension costs of £100,000–£150,000 all-in can represent strong return on investment, particularly in markets where 4-bed family homes command a significant premium over 3-bed properties.
Summary: Two Storey Extensions in London 2026
- Costs £72k–£220k for construction inc VAT depending on footprint size and specification.
- Planning permission almost always required in London. The 7m rear boundary rule eliminates PD eligibility for most properties.
- Party wall agreement almost always needed for raising shared walls and new excavation.
- Cheaper per m² than single storey over the same footprint. 50–70% more cost gives you 100% more floor area.
- Allow 14–20 months from architect appointment to completion including planning and party wall.
- Strong ROI in London when the upper floor adds a bedroom. Going from 3 to 4 bedrooms in Zones 1–3 typically adds £150k–£300k in value.
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