How Much Does an L-Shaped Extension Cost in 2026? (UK)
An L-shaped (side-return-plus-rear) extension in 2026 UK costs £58,000 to £135,000 on a typical Victorian or 1930s semi. The headline number hides three big swing factors: internal floor area added, glazing as % of envelope, and the kitchen specification that almost always lands inside the new space. London adds 18-28%; Scotland and the North East run 8-14% below the national average.
2026 L-shaped extension cost breakdown
| Scenario (3-bed semi) | Internal area added | Build cost (2026) |
|---|---|---|
| Modest side-return + 3m rear, standard finish | 22 sqm | £58,000-£72,000 |
| Full side-return + 4m rear, mid finish, kitchen included | 30 sqm | £78,000-£95,000 |
| Wraparound with bifolds + 4m roof lantern, high finish | 36 sqm | £102,000-£128,000 |
| London zones 1-3 with structural glass and architect finish | 36 sqm | £125,000-£165,000 |
Where the money actually goes
On a mid-spec £88,000 L-shaped extension the cost split is roughly: shell and structure 38%, roof and glazing 18%, kitchen and joinery 17%, mechanical and electrical 12%, finishes and decoration 8%, fees and warranties 7%. The biggest single line item is almost always the steelwork holding up the corner where the two new walls meet the original house.
Five factors that swing the price £15,000+ either way
- Steel quantity and complexity. An L-shape needs more steel than a square rear extension because two openings meet at a corner. Budget £6,500-£11,000 for steel + padstones on a typical L.
- Glazing-to-wall ratio. A 4m bifold runs £6,500-£9,500 fitted. A 3m x 2m roof lantern adds another £3,800-£5,200. Glass beats brick on impact and cost.
- Kitchen specification. A trade-supplied Howdens kitchen with quartz worktop lands around £14,000-£19,000 fitted. A bespoke joiner kitchen with stone runs £28,000-£55,000.
- Floor specification. Engineered oak with UFH adds £110-£160 per sqm. Large-format porcelain with UFH costs the same but lasts longer in a kitchen.
- Drains diversion. If the existing soil stack runs through the new footprint, expect £2,400-£4,800 to divert. Establish this at survey, not at strip-out.
Planning and Building Regs in 2026
Most L-shaped extensions on semis and detached homes fit under Permitted Development if the rear depth stays within 6m (semi/terraced) or 8m (detached) under the Larger Home Extension prior-approval route, and the side-return does not exceed half the original house width. Terraced houses cannot use the 6m/8m route. Conservation areas and Article 4 directions remove PD entirely. Building Regs full plans submission is non-negotiable; budget £1,800-£2,800 in fees including inspections.
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