Published 25 April 2026 · Updated 14 May 2026
By Jon Thompson, Director — Living Design & Build Ltd, FMB-member builder, Bridgnorth
The question we get asked most at first site visits is: should I build a single-storey or a two-storey extension? Here's how we think it through with every client.
The honest cost comparison
A common myth is that two-storey extensions cost twice as much as single-storey. They don't. The foundations, roof, scaffolding and groundworks are shared between the two floors, so the second storey only adds about 60% to the cost of the first.
- Single-storey rear extension, 25m²: £50,000–£70,000
- Two-storey rear extension, 50m² total: £85,000–£130,000
- Cost per m² (single-storey): £2,000–£2,800
- Cost per m² (two-storey): £1,700–£2,600
Two-storey extensions are nearly always better value per square metre. The question is whether you actually need the extra space upstairs.
When single-storey is the right call
- You want a big open-plan kitchen-diner with garden access
- Your upstairs already has enough bedrooms and bathrooms
- You're on a tight budget and need to avoid scaffolding/upper-floor disruption
- Your roofline or planning constraints make two-storey impractical
- You want bi-folds, sliding doors or a lantern roof — these read best on a single storey
When two-storey makes more sense
- You need a 4th or 5th bedroom (huge resale value in Shropshire and West Midlands suburbs)
- You want a master suite with en-suite and walk-in wardrobe
- Your kitchen is already a decent size but the family is growing
- You're staying in the house long-term and want to maximise per-m² spend
Planning and permitted development
Single-storey rear extensions up to 4m on detached houses (3m on semi/terraced) usually fall under permitted development. Two-storey extensions almost always need full planning permission — there are no permitted development rights for upper-floor rear extensions on most properties. Add 8 weeks and £258 to the timeline.
Disruption — what to expect
Single-storey extensions are cleaner. The work stays largely outside the existing house until the connecting wall comes down at week 8 or 9. Two-storey extensions involve scaffolding up against the house for 12+ weeks, dust through the upstairs windows for a fortnight while the roof opens up, and longer kitchen downtime if the works connect to the kitchen.
Frequently asked questions
Can I extend up later if I do a single-storey now?+
Sometimes, but it's expensive. The single-storey foundations and walls may not support a second floor without strengthening. If there's any chance you'll want to extend up later, build the single-storey foundations to take a second storey from the start — it adds about £3,000–£5,000 and saves £15,000+ if you ever do.
Do two-storey extensions affect my neighbours' light?+
Possibly. Two-storey extensions trigger the 45-degree daylight rule from neighbouring windows. We always check this at design stage and adjust the footprint if needed. Party-wall notices are also required.
Honestly, the right answer depends on what you actually need from the space. We'll happily come and walk through both options on your specific property — single-storey, two-storey, costed properly, with a real opinion. Free, no obligation.