Single Storey vs Double Storey Extension Costs
Choosing between a single storey and double storey extension is one of the biggest decisions in any home improvement project. Both have clear advantages depending on your needs, budget and property. Here is a detailed comparison covering costs, value, planning rules and build times.
Cost Comparison
A single storey extension typically costs £1,500-£2,500/sqm. A double storey extension costs around £1,800-£3,000/sqm total — but because the upper floor shares foundations and the roof, the cost per square metre of usable space is significantly lower. A 20 sqm single storey might cost £40,000, while a 40 sqm double storey (same footprint) might cost £60,000-£70,000 — only 50-75% more for double the space.
Detailed Cost Breakdown
For a typical 20 sqm footprint: Single storey foundations £4,000-£6,000 vs double storey £5,000-£7,000 (only slightly more). Walls £6,000-£10,000 vs £12,000-£18,000 (double the material). Roof £4,000-£8,000 vs £4,000-£8,000 (same cost). Steelwork £2,000-£4,000 vs £3,000-£6,000. The big savings come from shared foundations and roof — these fixed costs are split across twice the floor area.
Value Added to Your Property
Double storey extensions typically add more value to your property because they create additional bedrooms or bathrooms upstairs. An extra bedroom with en-suite can add £20,000-£40,000+ to your property value depending on location. A single storey kitchen-diner extension also adds strong value, particularly in family-oriented areas where open-plan living is in high demand. Either type typically recoups 50-70% of its cost in added property value.
Planning Permission Differences
Single storey rear extensions often fall under permitted development — up to 3m from the rear wall (or 6-8m under prior approval) without needing planning permission. Double storey extensions face stricter rules: they must not extend beyond 3m from the rear wall and the upper floor must be at least 7m from the rear boundary. Many double storey extensions require full planning permission (£258, 8-week decision). This adds both cost and time to the project.
Build Time Comparison
A single storey extension takes 10-14 weeks on site. A double storey takes 14-20 weeks. The additional time covers scaffolding, upper floor construction, staircase modifications, and extra first and second-fix work. If minimising disruption is a priority, a single storey extension is quicker and less intrusive — particularly as bedrooms upstairs remain unaffected during most of the build.
Structural Considerations
Double storey extensions require more substantial foundations and structural steelwork. Upper floor joists, load-bearing walls, and the connection to the existing first floor all add structural complexity. The existing house may need strengthening where the new extension meets the old building. A structural engineer must design the steelwork and connections, adding £500-£1,500 to professional fees.
Which Is Right for You?
Choose single storey if you primarily need ground-floor living space — a bigger kitchen, a family room, a home office, or an open-plan living area. Choose double storey if you need both ground-floor space and additional bedrooms or bathrooms upstairs. If you are unsure, consider whether your family is growing, whether you work from home, and what the property market in your area values most.
Common Mistakes When Choosing Between Single and Double Storey
The most common mistake is choosing purely on cost without considering long-term needs. Families who build a single storey and later realise they need bedrooms end up spending more on a second project. Equally, building a double storey when you only need kitchen space means paying for rooms you do not need. Other mistakes include not checking planning restrictions before committing to a design, and not considering how a double storey extension will affect light and privacy for both you and your neighbours.
Extensions in Worcestershire and the West Midlands
Across Worcester, Malvern, Bromsgrove and the wider West Midlands, both single and double storey extensions are popular with families. The mix of semi-detached properties, Victorian terraces and detached homes means the best option varies by property type. Conservation areas around Worcester city centre, Pershore and Malvern may restrict double storey designs. Clay soils common across Worcestershire can affect foundation costs for both types. MTD Build is based near Worcester and advises on the best approach during a free consultation.
Getting Expert Advice
The right choice depends on your specific property, budget and needs. Use our AI estimate generator to compare rough costs for both options. Upload a photo to our visualiser to see what each option might look like on your house. Or book a free consultation to discuss your project in detail — we will help you weigh up the options based on your property and priorities.
Frequently Asked Questions
Is a double storey extension worth the extra cost?
Usually yes, if you need the space. The upper floor adds only 50-75% to the cost while doubling your usable area. It is the most cost-effective way to maximise space.
Do I need planning permission for a double storey extension?
Many double storey extensions require full planning permission. They must not extend beyond 3m from the rear wall and the upper floor must be at least 7m from the rear boundary.
Which adds more value — single or double storey?
A double storey extension typically adds more value, especially when the upper floor creates an additional bedroom with en-suite. However, a high-quality single storey kitchen-diner also adds strong value.
How much more does a double storey extension cost than single storey?
A double storey extension typically costs 50-75% more than a single storey on the same footprint. A 20 sqm single storey costing £40,000 would be around £60,000-£70,000 as a double storey.
Is a double storey extension more disruptive to live with?
Yes. It involves more scaffolding, takes 4-6 weeks longer, and may affect upper-floor bedrooms during construction. Single storey extensions are generally less disruptive.
Which type of extension is most popular in Worcestershire?
Both are popular. Single storey kitchen-diner extensions are the most common, but double storey extensions are increasingly popular for growing families needing additional bedrooms.