A double-storey — or two-storey — extension adds living space on both the ground and first floors in a single build programme. Because the expensive groundwork (foundations, structural steelwork, and external walls) is shared across two floors, the cost per square metre of combined floor area is significantly lower than building a single-storey extension alone. If you need both a larger kitchen downstairs and a new bedroom or bathroom upstairs, a double-storey extension is almost always the most cost-effective route.
What Is a Double-Storey Extension?
A double-storey extension is an extension to a house that adds rooms on both the ground floor and the first floor. It can be built to the rear, to the side, or as a combination of both. The upper floor is structurally supported by the walls of the ground-floor extension, sharing the same foundations and roof. In practice, this means the incremental cost of the second storey — once foundations, steelwork, and external walls are already included in the ground-floor design — is relatively modest.
Common configurations:
- Rear double-storey: extends the back of the house on both floors. Ground floor gains a larger kitchen-diner; first floor gains a bedroom, dressing room, or bathroom.
- Two-storey side extension: extends into the side garden of a semi-detached or detached house. Creates a new side return on the ground floor and a bedroom on the first floor. Often changes a 3-bed semi into a 5-bed home.
- Rear and side wrap: combines a ground-floor side return with a double-storey rear addition. A complex but highly effective project type.
Cost: Double vs Single Storey
The economic case for going double rests on shared costs. Consider a 5 m × 4 m extension footprint:
| Cost element | Single storey only | Double storey |
|---|---|---|
| Foundations (strip/pad) | £8,000–£14,000 | £8,000–£15,000 |
| Ground-floor walls, structure | £14,000–£22,000 | £14,000–£23,000 |
| First-floor structure (timber frame/concrete slab) | — | £8,000–£14,000 |
| First-floor walls | — | £10,000–£16,000 |
| Roof (pitched or flat) | £9,000–£16,000 | £10,000–£18,000 |
| Internal fit-out (both floors) | £12,000–£20,000 | £22,000–£38,000 |
| Windows and doors | £5,000–£10,000 | £9,000–£18,000 |
| Professional fees, building control | £5,000–£10,000 | £7,000–£13,000 |
| Total (London & SE) | £53,000–£92,000 | £88,000–£137,000 |
| Total (Midlands / North) | £38,000–£66,000 | £62,000–£97,000 |
| Floor area created | 20 m² | 40 m² |
| Cost per m² (London) | £2,650–£4,600 | £2,200–£3,425 |
The double-storey option creates twice the floor area for roughly 1.5–1.65× the cost of the single-storey version — a clear efficiency gain.
Permitted Development Rules for Double-Storey Extensions
Two-storey extensions have more restrictive Permitted Development rights than single-storey versions.
Under Schedule 2, Part 1, Class A of the GPDO 2015 (England), a double-storey rear extension under PD must:
- Not extend more than 3 m beyond the rear wall of the original house (versus 6 m or 8 m for single storey)
- Not be within 7 m of the rear boundary of the curtilage
- Not exceed the height of the existing roof ridge
- Not have any windows on a wall or roof slope facing a side boundary (within 7 m of the boundary)
- Use materials of similar appearance to the existing dwelling
Side extensions under PD must not result in a building that is wider than half the width of the original house — a common constraint on semi-detached properties.
In practice: most double-storey rear extensions to terraced and semi-detached houses that extend more than 3 m, or that require planning for any other reason, will need a full householder planning application (fee: £258 in England as of 2026). Double-storey side extensions on semi-detached houses almost always require planning permission because they change the street scene. Allow 8–13 weeks for the application process; approval rates for well-designed double-storey extensions on residential streets are typically 85–90%.
Conservation areas and Article 4 direction areas remove PD rights entirely — check with your Local Planning Authority.
Structural Considerations
A double-storey extension is more structurally demanding than a single-storey addition:
Foundations: must be designed to carry the load of two storeys. In clay soils (common across London and the Midlands), trench-fill foundations are typically 900–1,200 mm deep; near trees, this may increase to 1,500–2,000 mm. A structural engineer will specify based on a site investigation.
Ground-floor slab: typically a reinforced beam-and-block floor or a 150 mm concrete slab on hardcore, designed to carry both the internal loads and the first-floor wall directly above.
Ground to first-floor structure: the intermediate floor is usually 220–250 mm engineered timber joists (e.g. I-joists) or a 150 mm reinforced concrete slab where acoustic separation is required (between living space below and bedroom above).
Steel beams: two sets of steels are usually required — one to span the opening in the existing ground-floor rear wall (typically a 203 × 133 or 254 × 146 UB) and one to support the new first-floor wall where it bears over the extended rear ground-floor space.
Roof: pitched roofs (typically at 30–40°) are preferred by planners on two-storey extensions; they relate better to the ridge line of the existing house. A flat or catslide roof may be accepted on rear extensions out of public view.
Structural engineer fees for a double-storey project: £1,200–£2,500 in 2026.
Party Wall Act
Almost all double-storey extensions on terraced and semi-detached houses trigger the Party Wall Act 1996. Both the ground-floor and first-floor construction will be within 3 m of the neighbour’s foundations (Section 6 notice) and works to or near the shared wall (Section 2 notice) are common.
Serve notices at least two months before planned construction start. If your neighbour dissents (or doesn’t respond within 14 days), a party wall award is required — budget £900–£2,500 in surveyors’ fees and 6–10 weeks of delay.
Design Ideas for Double-Storey Extensions
Ground floor: most commonly an extended kitchen-diner across the full width of the rear, with bi-fold or sliding doors to the garden. Rooflights are often not possible on double-storey extensions (the first floor sits where the roof would have been), so ensure adequate glazing in the rear elevation.
First floor options:
- Master bedroom with en-suite: the most popular choice. A 4 × 5 m footprint creates a spacious double bedroom (15 m²) plus an en-suite (5 m²) with separate shower enclosure and freestanding bath.
- Children’s bedroom: useful when growing families need a 4th or 5th bedroom. A 3 × 4 m extension creates a generous single or small double.
- Bathroom or shower room: if the existing house has only one bathroom, a new first-floor extension can accommodate a full family bathroom.
- Study or home office: an upstairs room with a window overlooking the garden makes an excellent home office — separated from household noise by a door.
Two-storey side extension: on a 1970s or 1980s semi-detached house with a wide plot, a two-storey side extension can transform the home. The ground floor typically creates a garage, utility room, or dining room; the first floor creates one or two new bedrooms. This type of extension changes the street-facing silhouette of the house, so planning authorities expect the design to complement the existing elevations carefully — matching brickwork, roof pitch, and window rhythm.
Building Regulations Requirements
All double-storey extensions require Building Regulations approval. Key parts:
| Part | Requirement |
|---|---|
| Part A | Structural design of foundations, steel beams, intermediate floor, and roof |
| Part B | Fire safety: escape windows from all first-floor rooms; smoke detection; fire doors where required |
| Part L | Wall U-value ≤ 0.28 W/m²K; roof ≤ 0.18 W/m²K; glazing ≤ 1.6 W/m²K |
| Part F | Mechanical extract ventilation in new bathrooms; trickle vents in new windows |
| Part K | Stair design if a new stair is formed (min. going 220 mm, max. rise 220 mm) |
| Part P | New first-floor electrical circuits certified |
Submit a Full Plans application before construction commences. Building control fees for a double-storey extension: £1,100–£2,200 depending on project value and local authority.
Programme
| Stage | Duration |
|---|---|
| Architect design, planning application | 10–16 weeks |
| Building Regulations approval | 5–8 weeks (can overlap design) |
| Foundations and groundworks | 2–4 weeks |
| Ground-floor structure and walls | 3–5 weeks |
| First-floor structure and walls | 2–4 weeks |
| Roof structure and covering | 2–3 weeks |
| Windows, external doors, external finish | 2–3 weeks |
| First fix M&E, insulation, plasterboard | 3–4 weeks |
| Second fix, tiling, decoration | 3–5 weeks |
| Total site time | 18–28 weeks |
| Total elapsed time from first meeting | 9–14 months |
Double-storey extensions are complex projects. Appoint an architect with relevant extension experience; ask to see completed examples of two-storey work.
Is It Worth It?
RICS data and estate agent guidance consistently shows that adding a bedroom to a 3-bed semi increases its value by 8–15% — often more than the cost of a double-storey side extension. A £90,000 extension on a £450,000 property might add £55,000–£70,000 in value, with the lifestyle benefits on top. The economics are strongest in high-value areas and weakest in markets where prices are already at the ceiling for the street.
