Single-storey or double-storey extension: which is better?
A single-storey extension is simpler, quicker and often avoids a planning application, and suits a bigger kitchen or living space. A double-storey extension adds bedrooms too and costs less per square metre, about £1,550 to £3,000 per m² against £1,750 to £3,400 per m², but usually needs planning permission and takes longer to build.
Side by side
| Single-storey | Double-storey | |
|---|---|---|
| Guide cost per m² | £1,750 to £3,400 per m² | £1,550 to £3,000 per m² |
| Planning | Often permitted development or prior approval | Usually a householder application |
| Build time on site | 10–16 weeks | 16–24 weeks |
| Adds bedrooms | No (unless ground-floor bedroom) | Yes |
| Roof options | Flat roof, lantern, pitched | Pitched roof tied into the house |
When single-storey makes sense
If what you need is a bigger kitchen, dining or family space, and the bedrooms are fine, a single-storey extension gives you that with less disruption. Flat roofs and lanterns make the new room bright in a way a two-storey extension cannot.
When double-storey makes sense
If you need another bedroom or bathroom as well, building both floors at once is far cheaper than doing a single-storey extension now and going up later. The foundations are designed for two floors from the start.
Can I build single-storey now and add a floor later?
It is possible if the foundations and walls are designed for it, but it usually costs more overall and means a second round of disruption and planning. Tell us at the design stage if you might want to go up later.
This guide is general information, not legal or professional advice for your property. Rules can change; we check the current position for your address at the survey.