Comparison
Single-storey vs two-storey extensions
Both add space. They differ sharply on cost per square metre, planning risk, disruption and what they do to your neighbours' daylight.
Written bySusan Fairweather, Extension Design Specialist
Typical 20 m² build
£40k – £60k · 12 weeks
Glasgow · Scotland
180+
Glasgow extensions delivered
Warrant & completion certificate included
A single-storey extension adds ground-floor living space at the lowest total cost and the lowest planning risk. A two-storey extension adds roughly twice the floor area for around 60 to 75 per cent more cost, because foundations and roof are shared, but carries greater planning risk, greater structural demand and significantly more disruption to the existing house.
Head to head
| Factor | Single-storey | Two-storey |
|---|---|---|
| Typical build cost | £40,000 – £70,000 | £75,000 – £115,000 |
| Cost per m² | £2,000 – £2,800 | £1,900 – £2,600 |
| Space gained | One room, ground floor | Living space plus bedroom or bathroom |
| Planning risk | Often permitted development | Usually a full application |
| Time on site | 10 – 16 weeks | 16 – 26 weeks |
| Disruption | Ground floor and garden | Whole house, including roof and upstairs walls |
| Can you live in the house? | Usually yes | Often yes, with weeks of significant upheaval |
When a single-storey extension is the right answer
- The problem is the ground floor — a cramped galley kitchen, no dining space, no connection to the garden.
- You already have enough bedrooms for the household and the street.
- Neighbouring windows sit close to the boundary and a two-storey mass would trigger daylight objections.
- You want to stay in the house during the works with minimal upheaval.
- Budget is the binding constraint and the project must complete in one phase.
When a two-storey extension earns its cost
- You need bedrooms as much as living space — a growing family, or a home office that is permanently occupied.
- The property is detached or semi-detached with a side plot wide enough to take the footprint.
- The street's ceiling price supports the resulting value, which matters most in Bearsden, Newton Mearns, Giffnock and the West End.
- The existing roof already needs work, so scaffolding and roofing costs can be shared.
The hidden cost of going up
A two-storey extension means the existing upstairs is opened up. Bedrooms lose walls, the roof is cut into, and heating, plumbing and electrics upstairs are all disturbed. Budget for redecoration across rooms you were not planning to touch — it is the most commonly underestimated line on a two-storey job.
The middle path: one and a half storeys
Where a full second storey is refused or unaffordable, a single-storey extension with a raised roof and rooflights, or a dormer added to the existing roof over the extended footprint, can deliver much of the benefit. This route keeps the ridge line low, which is often exactly what a planning officer is asking for.
Future-proofing a single-storey build
If a second storey is a possibility later, design the foundations and ground-floor walls for the eventual load now. The additional cost at the time of building is modest. Retrofitting the capacity means underpinning, which is not.
Frequently asked questions
Is a two-storey extension better value than a single-storey?
Per square metre, yes. The foundations, groundworks and roof are shared between two floors, so the second storey typically adds 60 to 75 per cent to the cost while doubling the floor area gained.
Is a two-storey extension harder to get approved?
Usually. A two-storey extension has a far greater impact on neighbouring daylight and privacy, so it is more likely to require a full planning application and more likely to attract objections in dense Glasgow streets.
Can you add a second storey to an existing single-storey extension?
Only if the existing foundations and walls were designed for the load, which older extensions rarely were. A structural engineer must assess it, and underpinning is common.
Which adds more value in Glasgow?
A two-storey extension that adds a fourth bedroom and second bathroom usually adds more value than a single-storey kitchen extension of the same cost, provided the street supports the resulting price.
Weighing up up or out?
Tell us about your property and what you want to gain. We will talk you through the realistic cost, the planning route and the build sequence — no obligation.