Single vs double-storey extension: which is right for you?
If you’re weighing up a single-storey extension against going up two storeys, you’re asking the right question early — and it’s one of the biggest decisions you’ll make. The answer shapes your budget, your planning route, the disruption to your family, and what your home is worth at the end. Here’s how we’d talk it through over a cup of tea at your kitchen table.
The short version
A single-storey extension adds ground-floor space — usually a bigger kitchen-diner, family room or utility. A double-storey extension does the same downstairs but stacks a room on top, typically an extra bedroom and often an en-suite.
The headline that surprises most people: a double-storey extension is cheaper per square metre than a single-storey one. That doesn’t mean it costs less overall — it costs more in total because you get more space. But pound-for-pound, the second floor is the bargain.
Why double-storey is cheaper per square metre
It comes down to where the money goes. The expensive bits of any extension are the foundations and the roof. With a single storey, you pay for both to gain just one floor of usable space.
Go up a second storey and those same foundations and that same roof now serve twice the floor area. You’re adding walls, a floor structure and finishes — but you’re not digging twice or roofing twice, so the fixed costs are spread further.
As a rough guide, a single-storey extension in Bristol tends to land around £2,000–£3,000 per square metre for a good standard of finish, while a double-storey works out meaningfully lower per square metre. These are ranges, not promises — the real figure depends on ground conditions, access, the spec you choose (bi-fold doors, underfloor heating, kitchen quality) and how the existing house is built. Be wary of anyone quoting a confident exact price before seeing your home.
What about the value it adds?
Extra bedrooms and bathrooms are what move a property up a price bracket, especially in family areas like Bishopston, Redland and Horfield where buyers want that third or fourth bedroom. A double-storey extension that turns a two-bed into a three-bed can add more value than a single-storey that “only” enlarges a kitchen — even though kitchens sell homes emotionally.
That said, value isn’t only about resale. If you’re staying put for ten years, the right answer is the one that makes daily life better — a generous kitchen-diner you actually live in beats a spare bedroom you barely use.
A word of caution on over-development: there’s a ceiling to what any street will support. Pushing a modest terrace beyond what the area’s prices justify can mean you don’t get your money back. We’ll always give you our honest view on this.
Planning and permitted development
This is where the two routes genuinely differ, and it matters.
Permitted development
Many single-storey rear extensions can be built under Permitted Development (PD) — your automatic right to extend within set limits, without a full planning application. You may still need to notify the council under the Neighbour Consultation Scheme for larger rear extensions, but it’s a lighter touch.
Double-storey extensions are more restricted under PD. There are tighter rules on how far they project, how close they sit to boundaries, and matching materials — and in practice many need full planning permission. If your home is in a conservation area (parts of Clifton, Cliftonwood and Montpelier, for example) or is listed, PD rights are often removed entirely and you’ll need permission either way.
A quick reality check on terms:
- Building control is separate from planning. Every extension, PD or not, needs building control sign-off to confirm it’s structurally safe and meets regulations.
- The Party Wall Act applies if you’re building near or on a shared boundary — common on Bristol’s terraces. You may need a written party wall agreement before work starts. It’s routine, but it takes time, so plan for it.
What to ask before you commit
- Will this need full planning permission, or does it fall under PD?
- Are we in a conservation area, and are PD rights restricted here?
- Will it trigger the Party Wall Act with either neighbour?
- What’s the realistic timeline once permissions and notices are factored in?
When each one makes sense for a family
Choose single-storey if: you mainly need better living and cooking space, you want to keep things simpler and faster, your budget is firmer, or planning constraints make going up difficult. It’s the classic move for a growing family who love their bedrooms but feel cramped downstairs.
Choose double-storey if: you need more bedrooms or bathrooms (another child on the way, ageing parents, working from home), you’re already investing in foundations so want to maximise the return, and the plot and planning rules allow it. It’s the better long-term play when space, not just layout, is the problem.
A few homeowners find a loft conversion does the upstairs job more cheaply than a second storey — worth comparing if all you need is one more bedroom. And while the walls are open is the sensible moment to think about our heating services, as extending often means resizing radiators or rethinking the boiler.
A straight bit of advice
There’s no universally “better” option — only the one that’s right for your home, your family and your budget. The cheapest per square metre isn’t always the wisest spend, and the biggest isn’t always the best.
If you’d like a proper steer, we’re happy to come round, have a look and talk through both routes with no pressure. You can see our recent work and the areas we cover, or simply get a free quote — a no-obligation home visit and a fixed written price, so you know exactly where you stand before deciding anything.