Double Storey Extension Cost Guide 2026
A double storey extension adds space on two levels, making it significantly better value per square metre than building two separate single storey extensions. You share foundations, site setup, and roof costs across both floors, so the cost per m² is typically only 10–15% more than a single storey.
Two storey extensions are more likely to need full planning permission, and the structural requirements are greater — deeper foundations, more steelwork, and full-height scaffolding. This guide provides a 2026 trade-by-trade cost breakdown to help you plan and evaluate quotes.
Cost summary
| Size | Basic | Standard | Premium |
|---|---|---|---|
| 40m² (20m²/floor) | £31,480 | £56,200 | £87,120 |
| 60m² (30m²/floor) | £47,220 | £84,300 | £130,680 |
| 80m² (40m²/floor) | £62,960 | £112,400 | £174,240 |
Costs shown are direct trade costs (national average). Apply regional factors below for your area. Main contractor route adds 20–35%.
Trade-by-trade breakdown
Standard specification, mid estimate. Per-sqm rates for direct trades.
Regional cost variations
| Region | Factor | Adjusted cost |
|---|---|---|
| Inner London | ×1.35 | £1,897/m² |
| Outer London | ×1.22 | £1,714/m² |
| South East | ×1.08 | £1,517/m² |
| South West | ×1.00 | £1,405/m² |
| East Midlands | ×0.92 | £1,293/m² |
| North West | ×0.90 | £1,265/m² |
| North East | ×0.85 | £1,194/m² |
Multiply the national average cost by the regional factor for your area.
Key cost factors
Structural complexity
Two storey extensions require more substantial steelwork, especially where ground floor walls are opened up. Large openings with rooms above need significant RSJ support, adding £2,000–£6,000.
Internal staircase
If the extension includes a new internal staircase (rather than connecting to the existing landing), budget £3,000–£6,000 for the staircase, structural alterations, and balustrade.
Planning permission
Two storey extensions almost always require full planning permission. They must not extend beyond the rear wall of the original house by more than 3m, and must be at least 7m from the rear boundary.
Foundation depth
Two storey loads require deeper foundations than single storey. On clay soils or near trees, foundations may need to be 1.5–2.5m deep, significantly increasing groundwork costs.
Party wall implications
Two storey extensions near boundaries have greater impact on neighbours' light and privacy. Party Wall Act agreements are almost always required and can cost £1,000–£2,000 per neighbour if disputes arise.
Frequently asked questions
Do I need planning permission for a two storey extension?
Almost always, yes. Two storey extensions rarely qualify as permitted development. You'll need to submit a full planning application, which costs £206 (householder) and takes 8–12 weeks for a decision. Pre-application advice is recommended.
How much more does a two storey extension cost than single storey?
A two storey extension costs roughly 10–15% more per m² than single storey, but you get twice the floor area. For example, a 60m² double storey (30m² per floor) costs around £84,000 at standard spec — far less than two separate 30m² extensions.
How long does a double storey extension take?
A typical two storey extension takes 14–22 weeks from groundworks to completion. Larger or more complex projects with significant structural alterations may take 6 months or more.
Do I need a structural engineer?
Yes. A structural engineer is essential for a two storey extension to design foundations, steel beams, and first floor structure. Their calculations ensure the build is safe and meets Building Regulations. Fees typically range from £1,000–£3,000.
Will a double storey extension add more value than single storey?
Generally yes. A two storey extension adding a bedroom and bathroom upstairs alongside living space downstairs typically adds 15–25% to property value, compared to 5–10% for a single storey. The extra bedroom is particularly valuable in family-sized homes.
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