Two-storey extensions: more room, tighter rules
Roughly a quarter of the extensions people ask us about are two storeys. It is the option that actually solves a bedroom shortage — and the permitted development limits get noticeably less generous the moment you go above one floor.
The case for going up as well as out
Three quarters of the enquiries we see are single storey, and for a lot of households that's the right answer: the kitchen is the problem, the garden can spare 3m, done. But a single-storey extension cannot fix a house that's one bedroom short, and no amount of open-plan ground floor changes how many people can sleep there.
Two storeys costs less per square metre than one, because you're getting a second floor of space out of the same foundations and roughly the same roof. That's the strongest argument for it. If you're already committed to digging, the marginal cost of the upper floor is lower than the average cost of the lower one.
The counterweight is that it's a bigger intervention in every sense — more disruption, more likely to need permission, more visible to the neighbours, and much more likely to attract an objection.
What permitted development allows
Class A does permit extensions of more than one storey, but the conditions tighten sharply:
| Condition | Limit |
|---|---|
| Depth beyond the original rear wall | 3m maximum |
| Distance from the rear boundary | At least 7m must remain |
| Roof pitch | Must match the existing house as far as practicable |
| Side-facing upper windows | Obscure-glazed, and non-opening below 1.7m from the floor |
| Balconies, verandas, raised platforms | Not permitted |
| To the side of the house | Not permitted — side extensions must be single storey |
| Designated land | Not permitted |
The 7m rear boundary condition is the one that rules most people out. On a typical suburban plot with a 12m garden, a 3m two-storey extension leaves 9m and you're fine. On a shorter garden, or a house already extended once, you're into a planning application whatever the depth.
And note the side restriction. A two-storey side extension — the classic way to add a bedroom over a garage — is never permitted development. That one always needs an application.
What it costs
Indicatively £50,200 to £90,339, averaged across 519 UK towns, from BestBuilders' 2026 regional cost model. That's roughly double the single-storey range of £24,928 to £50,200 for substantially more than double the floor area, which is the per-square-metre point made above. As always these are modelled ranges, not quotes, and the region you're in moves them more than most people expect. BestBuilders' cost page has the per-type breakdown; it's a sister site of ours.
Budget separately for the things a second storey adds that a single storey doesn't. Structural steel is heavier. You may need to upgrade existing foundations to carry the extra load, which is invisible in a per-square-metre figure and can be five figures on its own. And the first-floor layout usually forces changes to the existing landing and stairs, so the work spills into rooms you weren't planning to touch.
The neighbour question
Two-storey extensions generate most of the householder objections councils receive, and the reasons are consistent: overlooking, and loss of light. A first-floor window looking into next door's garden is the single most reliable way to attract an objection, and the obscure-glazing condition in Class A exists precisely because of it.
If you're going the application route, design that out before you submit rather than after the objection lands. Rooflights instead of side windows, high-level glazing, or simply putting the new window on the other elevation costs nothing at the drawing stage and a great deal at the appeal stage.
The 45-degree rule most councils apply is worth reading up on in your own authority's guidance — it is local policy rather than national law, and the way it is drawn varies.
Frequently asked
Where the rules come from
Class A, Part 1, Schedule 2 of the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended, and the government's technical guidance on permitted development for householders. The 45-degree and 25-degree daylight tests are local planning policy, published in your council's householder design guidance. England only.
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