Single vs Double Storey Extension Cost UK: Which Gives Better Value?
Adding an extension is one of the most effective ways to increase both your living space and your property's value. But should you build single storey or double storey? The upfront costs, planning requirements, build times, and value uplifts are meaningfully different — and the right answer depends on your property, your budget, and what your local planners will approve. This guide compares both options head to head.
Cost Comparison
Costs are quoted per square metre of floor area added. Note that a double storey extension adds floor area on two levels, so the total floor area gain is significantly larger than a single storey of the same footprint.
| Single Storey | Double Storey | |
|---|---|---|
| Cost per m² | £1,500–£2,200 | £1,800–£2,500 |
| Typical footprint | 20–30m² | 20–30m² |
| Total floor area added | 20–30m² | 40–60m² |
| Typical total build cost | £35,000–£65,000 | £70,000–£140,000 |
| London uplift | Add 20–30% | Add 20–30% |
Regional variation is significant. London and the South East consistently command the highest rates. The North West, Yorkshire, and the East Midlands are typically 15–25% cheaper than the national average. Always get at least three local quotes.
Why Double Storey Often Gives Better Value Per m²
At first glance, double storey costs more per square metre. But look at what you get for that extra spend:
- Foundations are largely shared: The foundations for a double storey extension need to be deeper and wider than for single storey, but you're not paying for two sets of foundations. A single excavation, single pour, and single structural frame serves both floors.
- The roof spans both floors once: A single storey has a roof covering 20–30m² of new floor area. A double storey has the same roof but it's covering 40–60m² of new floor area — effectively halving the roof cost per m².
- External walls serve both floors: Scaffolding, brickwork, insulation, and rendering is done in one mobilisation rather than two.
The result: the incremental cost of adding the second storey on top of an existing double storey footprint is often just £30,000–£50,000 more than stopping at single storey — but you gain 20–30m² of additional floor area.
Planning: A Critical Difference
Single Storey — Often Permitted Development
Under the General Permitted Development Order 2015 (GPDO) Class A, many single storey rear extensions don't need planning permission:
- Detached houses: Up to 4m depth without Prior Approval; up to 8m depth with Prior Approval (the Neighbour Consultation Scheme)
- Terraced or semi-detached houses: Up to 3m without Prior Approval; up to 6m with Prior Approval
- Must not exceed 4m in height or the height of the existing eaves
- Cannot cover more than 50% of the original garden area
Prior Approval (for the larger depths) requires a 42-day neighbour consultation period but is not full planning permission — it's a lighter-touch process.
PD rights are removed for listed buildings, in some conservation areas, and where an Article 4 Direction applies. Check with your local planning authority before assuming PD. For the full planning permission process for extensions, see our planning permission for house extensions guide.
Double Storey — Almost Always Full Planning
Double storey rear extensions fall outside permitted development in most cases. PD only allows two-storey additions under very specific conditions (same plane as existing rear wall, eaves and ridge match, no balconies, 7m from rear boundary) — conditions few real extensions meet.
Expect a full householder planning application, a fee of £258 in England (as of 2024), and an 8-week determination period. Planning applications for double storey extensions are generally approved where they respect the scale and character of the area and don't overlook or overshadow neighbours.
Allow an additional £800–£2,000 for architect drawings for the planning application.
Build Timeline
| Stage | Single Storey | Double Storey |
|---|---|---|
| Planning (if required) | N/A or 6–10 weeks PA | 8–12 weeks |
| Building Regs drawings | 2–4 weeks | 3–6 weeks |
| Build on site | 10–14 weeks | 18–24 weeks |
| Total from start to finish | 3–6 months | 6–10 months |
Double storey builds take longer because there are more stages: first-floor structure, first-floor joists, additional masonry lifts, and more complex finishing. Disruption to your home is proportionally greater.
Structural Considerations
Single storey: Relatively straightforward structural requirements. You'll need a structural engineer to design the steel beam (RSJ) that spans any opening into the existing house, plus standard strip or pad foundations. Structural engineer fees typically run £500–£1,500 for a standard single storey.
Double storey: More demanding on every front:
- Deeper foundations — typically 1m+ depending on soil type and proximity to trees
- Steel frame or engineered timber at first floor level
- Party wall notices (Section 2 for works on the party wall; Section 6 for excavation within 3m of a neighbour's foundations) — highly likely for semi-detached or terraced properties
- Structural engineer fees typically £1,500–£3,000
For semi-detached and terraced houses, the Party Wall Act is almost always triggered for a double storey — budget for a party wall surveyor at £700–£1,500. If your neighbour is unresponsive to a party wall notice, see our guide on what to do when a neighbour refuses a party wall notice.
Value Uplift: What Will You Get Back?
Extensions don't automatically return 100% of their cost in added value — it depends on the ceiling value of your street and whether your property is already priced at that ceiling. On a typical UK house valued at £300,000–£600,000:
| Extension Type | Typical Value Uplift |
|---|---|
| Single storey (kitchen/diner, 20–25m²) | 10–15% |
| Double storey (kitchen/diner + bedroom/bathroom, 40–50m²) | 15–20% |
On a £400,000 house:
- Single storey adds roughly £40,000–£60,000 in value — on a £45,000 build cost, that's strong ROI
- Double storey adds roughly £60,000–£80,000 in value — on a £100,000 build cost, the ROI is lower but you're also getting significantly more usable space
Important caveat: If your house is already near the ceiling value for your street, value uplift will be lower — expensive additions rarely push a house above the average street price by much. Your estate agent can give you a realistic ceiling figure.
The Decision Framework
Ask yourself four questions:
1. What's your budget? Single storey is accessible from around £35,000. Double storey typically requires £70,000 minimum. Be honest about contingency — budget 15% for unforeseen costs on any extension.
2. Do you need planning permission? If you're on a conservation area, a listed building, or your property has had PD rights removed, both options require planning. But single storey has more flexible PD routes, so check your permitted development position first.
3. What does your family actually need? If the goal is a larger kitchen or open-plan living, single storey delivers it. If you need an extra bedroom and bathroom as well, double storey is the only efficient solution — building single storey now and adding a second floor later is far more expensive than doing both at once.
4. What's the headroom in your property's value? If your £350,000 house has a ceiling of £600,000 on your street, a double storey makes sense. If the ceiling is £420,000, you'll spend more than you'll recover.
Bottom Line
Single storey wins on simplicity, lower upfront cost, and planning ease. Double storey wins on floor area per pound spent, long-term family practicality, and value uplift — provided you have the budget, the planning appetite, and the value headroom. For most homeowners with a growing family and a mid-range property, double storey delivers better long-term value if the budget can stretch.
For the full planning application process for extensions, see our planning permission for house extensions guide. If the Act is triggered for your project, our guide on party wall act: neighbour refusing explains the dispute resolution process, timelines, and who pays.
For a detailed 2025 cost breakdown for kitchen extensions specifically — size × quality tier cost tables, trade-by-trade breakdown, and glazing cost variables — see our kitchen extension cost UK 2025 guide. If you're funding your extension by releasing equity, our remortgaging for home renovation guide explains further advances, full remortgages, and second charges.