home extension UK

Quick Answer: A single-storey extension in the UK costs between £1,200-£2,000 per square metre in 2026, with typical projects ranging from £20,000-£60,000. Most single-storey rear extensions under 4m deep (3m for terraced/semi-detached) qualify for permitted development rights and don’t need planning permission. Two-storey extensions cost £1,800-£2,500 per m² and typically require full planning permission. Always check local planning portal and Building Regulations Part L (energy efficiency) requirements before starting. Understanding Home Extensions: What’s Right for Your Property Home extensions remain one of the most popular ways to add space and value to UK properties in 2026. Rather than facing the stress and expense of moving, homeowners are choosing to extend — and it’s easy to see why. According to government planning guidance, the right extension can add 15-20% to your property’s value whilst giving you the extra space your family needs. But get it wrong, and you’ll face planning headaches, cost overruns, and structural issues that’ll haunt you for years. Let’s cut through the confusion and give you the real-world knowledge you need to plan, budget, and execute a successful home extension. Types of Home Extensions: Comparing Your Options The type of extension you choose depends on your property layout, budget, and planning constraints. Here’s what most homeowners consider: Single-Storey Extensions Single-storey rear extensions are by far the most common. They’re ideal for creating open-plan kitchen-diners or extending living spaces into the garden. Most qualify for permitted development rights, meaning no planning permission needed. Typical size: 3m-6m depth, full width of property Cost: £1,200-£2,000 per m² (£24,000-£50,000 for typical 25m² extension) Build time: 2-4 months from groundworks to final plastering ROI: 10-15% property value increase The plastering work on a single-storey extension involves both internal walls and ceilings. You’ll typically need multi-finish plaster over plasterboard for walls, with particular attention to thermal efficiency where the extension meets the existing building. Two-Storey Extensions Adding a second floor gives you double the space but comes with higher costs and usually requires full planning permission. These work well for side returns or where you need both ground floor living space and additional bedrooms. Cost: £1,800-£2,500 per m² (£90,000-£150,000+ typical) Build time: 4-7 months ROI: 15-25% property value increase Planning: Almost always requires permission Side Return Extensions Popular in Victorian terraced houses, side return extensions fill in that narrow alley space between your house and the boundary. They’re perfect for enlarging kitchens without extending too far into your garden. Typical dimensions: 1.2m-2m wide, extending 3-5m back. Costs run £25,000-£50,000 for single-storey, significantly more if combining with rear extension. Wrap-Around Extensions Combining side and rear extensions creates an L-shaped wrap-around that dramatically transforms ground floor space. These are substantial projects requiring experienced builders and careful coordination between trades. Extension Type Typical Size Cost per m² Total Cost Range Planning Required? Single-storey rear 20-30m² £1,200-£2,000 £24,000-£60,000 Usually no (PD) Two-storey rear 40-60m² £1,800-£2,500 £72,000-£150,000 Yes Side return 8-15m² £1,500-£2,200 £12,000-£33,000 Often no (PD) Wrap-around 35-50m² £1,400-£2,100 £49,000-£105,000 Usually yes Loft conversion 25-40m² £1,600-£2,800 £40,000-£112,000 Depends on dormer Planning Permission vs Permitted Development: What You Actually Need This is where most homeowners get confused. The good news? Many extensions don’t require planning permission thanks to permitted development (PD) rights. Permitted Development Rules 2026 Under current Planning Portal guidelines, you can build without permission if your extension meets these criteria: Detached houses: Single-storey rear extensions up to 4m deep Semi-detached/terraced: Single-storey rear extensions up to 3m deep Height limits: Maximum 4m for flat roof, 4m to eaves for pitched roof Side extensions: No more than half the width of original house, single-storey only Materials: Should match existing building appearance Pro Tip: Even if your extension qualifies for PD, you still need Building Regulations approval. Many homeowners confuse these two separate requirements. Planning deals with visual impact and land use; Building Regs ensure structural safety, fire protection, and energy efficiency. When You Definitely Need Planning Permission You’ll need to submit a full planning application (£258 fee in 2026) for: Two-storey extensions of any size Extensions exceeding permitted development limits Properties in conservation areas or listed buildings Flats or maisonettes (PD doesn’t apply) Extensions that would result in over 50% ground coverage Any extension with balconies, verandas, or raised platforms Planning applications take 8-13 weeks typically. Budget an extra £1,500-£3,500 for architect drawings and £500-£1,200 for structural engineer input. Building Regulations: The Non-Negotiable Requirements Every extension requires Building Regulations approval — no exceptions. This ensures your extension is structurally sound, thermally efficient, and safe. The Building Control approval process costs £800-£1,500 for typical extensions and involves inspections at key stages: Foundation inspection: Before concrete pour Damp-proof course: Checking DPC levels and membrane placement Drainage: Before backfilling trenches Pre-plaster inspection: Insulation, electrical, heating roughed-in Final inspection: Completed work with all finishes Part L Energy Efficiency Requirements Part L regulations in 2026 are stringent. Your extension must achieve U-values of: Walls: 0.18 W/m²K or better Roof: 0.15 W/m²K Floor: 0.18 W/m²K Windows/doors: 1.4 W/m²K This typically means 100mm PIR insulation in walls, 150mm+ in roofs. The insulated plasterboard we install usually has 50-75mm insulation bonded to 12.5mm board. ⚠️ Warning: Don’t skip Building Control inspections. If you sell your property without a completion certificate, buyers’ solicitors will require retrospective approval (expensive) or indemnity insurance (which doesn’t actually confirm the work is safe). We’ve seen sales fall through over missing certificates. Real-World Extension Costs: The Complete Breakdown Let’s talk actual money. These are realistic 2026 costs based on current Kent and Southeast England pricing. Expect 10-15% less in Northern England, 15-25% more in London. Budget Breakdown for 25m² Single-Storey Extension Item Cost Range Notes Groundworks & foundations £4,000-£6,500 Strip foundations, 1m deep typically Brickwork & blockwork £5,500-£8,000 Includes DPC, lintels, airbricks Roof structure & covering £4,000-£7,000 Flat roof with warm deck insulation Windows & doors £2,500-£4,500 Bi-fold doors cost £2,000+ alone Electrics (first fix & second) £2,000-£3,500 New consumer unit if required: +£800 Plumbing & heating £2,500-£4,000 Underfloor heating adds £1,200-£2,000 Plastering (walls & ceiling) £1,800-£2,800 £35-£45/m² for board and skim Flooring