When clients first call about an extension, most have a single-storey kitchen extension in mind. And often that's the right call. But at the site visit I'll usually ask one question that changes the conversation: "Have you thought about going up as well as out?"
Here's why, and how to work out which is right for your home.
The economics: two floors share one roof
The most expensive parts of any extension are the groundworks and the roof. Foundations, drainage, steels, roof structure, tiling and weatherproofing account for a large slice of the budget — and here's the thing: a double-storey extension uses the same foundations and the same roof as a single-storey one.
That second floor is mostly walls, floor joists, windows and finishes — the cheap part. So while a double-storey extension costs more in total, the cost per square metre is significantly lower. As a rough guide from our own projects:
- A single-storey rear extension typically comes in between £55,000 and £85,000.
- A double-storey version of the same footprint typically comes in between £90,000 and £120,000 — not double the price, for double the space.
If you need an extra bedroom or bathroom as well as a bigger kitchen, building both at once is nearly always better value than doing them as two separate projects years apart.
When single-storey is the right answer
Value per square metre isn't everything. A single-storey extension is usually the better choice when:
- The ground floor is the whole problem. If the bedrooms are fine and it's the cramped kitchen that's driving you mad, don't buy space you don't need.
- You want the glass-box feel. Rooflights, lantern roofs and full-width glazing are what make modern kitchen extensions special — and they need a single-storey roof to sit in.
- Planning is tight. Many single-storey rear extensions fall under permitted development. Double-storey extensions almost always need a full planning application, and they're more likely to draw objections from neighbours about overlooking and overshadowing.
- Budget is fixed. A smaller project done to a high standard beats a bigger one built to a squeezed spec, every time.
When double-storey earns its keep
Go up as well as out when:
- You need a bedroom or bathroom anyway. This is the big one. If a loft conversion or a move is on the cards within five years, one double-storey build now is cheaper and less disruptive than two projects later.
- The house is unbalanced. Plenty of three-bed semis have a decent ground floor and one bathroom for five people. A double-storey extension fixes the actual problem.
- You're staying long-term. The extra disruption and planning time pay back over the years you'll live in the finished house.
What about value?
Agents will give you precise-sounding figures, but the honest version is this: extra bedrooms and bathrooms move a house up a price bracket; a better kitchen sells the house faster within its bracket. If your goal is resale value, double-storey usually wins. If your goal is loving the house you're in, the kitchen you use fifty times a day might matter more than a bedroom you don't currently need.
Deciding for your own home
The right answer depends on your house, your street and your plans, which is why I'd never quote either option over the phone. At a site visit we'll look at what the structure can take, what planning is likely to allow, and what actually fixes the problem you're living with. Then you get one itemised quotation for each option you're considering, so you can compare real figures instead of guesses.
Book a site visit, it costs nothing, and you'll get a straight answer either way.
Written by
Jason Berry
Director of J.Berry Extensions & Conversions. Every J.Berry project — extension or loft conversion — is personally surveyed, quoted and run by Jason Berry, from first site visit to handover.