Once you've decided to extend, one of the first big questions is whether to go out, or out and up. A single-storey extension adds space on one level; a double-storey one adds a room (or two) upstairs as well. Both can work well, and the right answer depends on your house, your budget, your neighbours and what you actually need.
Here's how the two compare on the things that matter.
Cost: which gives you more for your money?
The headline cost figure and the cost per square metre of usable space tell opposite stories — and it's worth understanding why.
Per square metre of footprint, a double-storey extension costs more — typically around 1.5 to 1.75 times as much as a single-storey one, because you're building two floors, a more substantial structure, and often more complex junctions with the existing house. For East Midlands projects, a sensible planning figure in 2026 is roughly £2,000 per m² for single-storey and £3,100 per m² of footprint for double-storey. (These are guide figures for budgeting, not a quote — specification varies enormously.)
Per square metre of floor area created, a double-storey extension costs less. The foundations and roof are shared across two floors, so you're getting double the floor area for much less than double the cost.
A worked example, for a 4m × 5m (20m²) footprint:
| Single-storey | Double-storey | |
|---|---|---|
| Floor area created | 20m² | 40m² |
| Approximate build cost | £40,000 | £62,000 |
| Cost per m² of floor area | £2,000 | £1,550 |
So the double-storey version costs about 55% more in total, for twice the floor space. If you genuinely need that second floor of space, it's usually the more economical way to get it.
Space: what do you actually need?
Cost per square metre is only useful if you need the space. Start from the problem you're trying to solve:
- A bigger kitchen or living space → a single-storey rear extension usually does it well.
- More bedrooms or a bigger bathroom → you need to go up, whether through a double-storey extension or a loft conversion.
- A mix of both → a double-storey extension can deliver a larger kitchen-diner downstairs and a bedroom suite above in one project.
Planning: single-storey is easier
This is where the two diverge most sharply.
- Single-storey rear extensions can often go ahead under permitted development (up to 3 or 4 metres, or 6 or 8 with prior approval). See Do You Need Planning Permission for a Rear Extension?
- Double-storey rear extensions are far more restricted under permitted development — a maximum of 3 metres, and no closer than 7 metres to the rear boundary — and in a conservation area, they always need planning permission.
- Double-storey side extensions always need planning permission.
A double-storey extension also has a much bigger effect on neighbours: potential loss of light, overlooking from new upstairs windows, and an overbearing appearance. Councils weigh these carefully, and neighbours can object. A well-designed scheme can overcome all of that — but it's a real consideration, and it adds time and fees to the project.
The effect on your garden
Every extension uses up garden. A double-storey extension gives you more internal space per metre of garden lost — one of its genuine advantages, particularly on smaller plots or semi-detached houses where garden space matters. If you want to keep as much outdoor space as possible, going up rather than out is often the smarter move.
Value: what does an extension actually add?
Be sceptical of any headline percentage ("an extension adds 20% to your home's value") — they're averages that hide enormous variation. A few things hold more reliably:
- Bedrooms and bathrooms drive value. In most markets, adding a bedroom, or turning a three-bed into a four-bed, tends to move a home into a higher price bracket. This is where double-storey extensions can pull ahead.
- A better kitchen and living space matters too — particularly the ground-floor layout, where a single-storey extension can transform how the whole house works.
- The cost-to-value relationship has a ceiling. Spending far more than comparable houses in your street are worth rarely returns what you put in. The more your house would be the most expensive on the road after the work, the more cautious you should be.
- Quality counts. Well-designed, well-built space that feels like part of the original house is worth more than a bolt-on.
The most reliable guide is local: ask two or three estate agents what a comparable house with the extra space sells for, compared with yours as it is now. That's real data, rather than a national average.
Don't forget the extras
Both options carry costs beyond the build: planning and building control fees, design and structural engineering, and a contingency. See How Much Does a Rear Extension Cost in 2026? for a breakdown.
So which is right for you?
Choose single-storey if you mainly want a bigger or better ground floor, you'd like the simplest planning route, or your budget is tight.
Choose double-storey if you need extra bedrooms or bathrooms, you want to protect garden space, or you want the best value per square metre of floor area and are comfortable with a more involved planning process.
If you're torn, the best next step is to have both options sketched and costed against your actual house. You can start with our online estimator, which prices both single and double-storey extensions, or get in touch to talk it through.