Work out what you are allowed to build — before you worry about the price
Most people planning an extension start with the same three questions: am I allowed to build it, what will it cost, and is it the right extension? This site answers them in that order, because the first one decides everything else.
The three questions, in the order that matters
1. Am I allowed to build it?
Most single-storey rear extensions are permitted development and need no planning application — but the limits are specific, and they differ for a terrace, a semi and a detached house. Start here before you pay anyone for drawings.
2. What will it cost?
Indicative 2026 ranges by extension type, and the handful of decisions — ground conditions, steel, glazing, drainage — that move the number far more than the floor area does.
3. Is it the right extension?
Rear, side, wraparound or two-storey. The cheapest per square metre is not always the one that solves the problem you actually have.
Where homeowners are when they start looking
These are answers people gave our own quote tool over the last 90 days, aggregated and anonymised. It is a demand signal from one website rather than a national survey, but it is a fair picture of who is asking:
| Question | Most common answer |
|---|---|
| How far along are you? | Still planning and researching — more than half |
| Planning permission | About a quarter do not know whether they need it |
| Drawings | Most have none yet |
| Type of extension | Rear, then side, then wraparound |
| Storeys | Single storey, by about three to one |
| Property | Semi-detached, then detached, then terraced |
If that is roughly where you are, the planning section is the right place to start.
Get quotes from extension builders near you
Tell us what you are planning and we will match you with up to three builders who cover your area. Free, no obligation, and you choose whether to go ahead.
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