Single-storey vs double-storey extension: which is better value?

The cost per square metre often favours going up — here is why

Buildcostr.ai · June 2026 · 5 min read

The short answer

A double-storey extension usually costs more in total but less per square metre than a single-storey one. You pay once for the foundations and the roof, then get two floors of space underneath. If you need the extra room, building up is often the better-value option.

What a single-storey extension costs

A single-storey extension typically costs £1,500 to £3,000 per square metre for the build, depending on specification and location. A 20m² addition usually lands between £30,000 and £60,000 before professional fees and VAT.

What a double-storey extension costs

A double-storey extension does not cost twice as much as a single-storey one of the same footprint. Because the expensive elements — foundations, groundworks and roof — are shared across two floors, the second storey often adds only 50–60% to the total cost while doubling the floor area. That is what brings the per-square-metre figure down.

Why going up can be better value

• Foundations are dug once for two floors of space
• The roof covers the same footprint whether there are one or two storeys
• Scaffolding and site setup are shared across the whole build
• You gain bedrooms or a bathroom without losing more garden

When single-storey makes sense

Single-storey still wins in plenty of cases — a tighter budget, a desire to keep the roofline low, more natural light from rooflights, simpler permitted-development rights, or simply not needing extra bedrooms. It is also less disruptive to build and live through.

Planning considerations

Double-storey extensions are more likely to need full planning permission and are more sensitive to overlooking and overshadowing of neighbours. Single-storey rear extensions can often be built under permitted development, subject to size limits. Always check with your local authority before committing.

Compare the two properly

The right answer depends on your specific design. An itemised cost plan lets you compare both options on real numbers rather than rules of thumb. Buildcostr.ai produces one in 30 minutes — see a sample or order yours.

For the full picture on extension pricing, read what a house extension really costs in 2026.