How much does a house extension cost per m2 in London 2025?

A single storey rear extension in London costs £2,200–£3,800 per m² in 2025. A standard 25m² extension costs approximately £55,000–£95,000 in outer London and £65,000–£115,000 in inner London. Costs vary by extension type, build quality and site conditions.

How much is stamp duty on a £300,000 house in 2025?

From 1 April 2025, a home mover buying a £300,000 property pays £3,750 in SDLT (0% on £125,000 + 2% on £125,000 + 5% on £50,000). A first-time buyer pays £0 as the property is under the £300,000 FTB nil-rate threshold.

How many bricks per square metre UK?

A standard half-brick wall (102.5mm thick) uses 60 bricks per m² with 10mm mortar joints. A full-brick wall (215mm) uses 120 bricks per m². Lightweight concrete blocks (440×215×100mm) use 10 blocks per m². Always add 5% for wastage.

What is the concrete mix ratio for a house foundation?

For house foundations and structural slabs, use a C25 mix with a ratio of 1:1.5:3 (cement:sharp sand:20mm aggregate). For heavy duty reinforced work, use C30 (1:1:2). For paths and driveways, a C20 mix (1:2:4) at 100mm depth is sufficient.

How much does a dormer loft conversion cost in London?

A standard dormer loft conversion in London costs £45,000–£80,000 for a 25–35m² conversion. An L-shaped dormer costs £60,000–£95,000. A Mansard loft conversion typically costs £70,000–£130,000+. Costs vary by specification, borough and whether a listed building premium applies.

What is SDLT and how is it calculated?

SDLT (Stamp Duty Land Tax) is a tax paid by property buyers in England and Northern Ireland. It is calculated on a tiered/banded basis — different rates apply to different portions of the purchase price. From April 2025, the nil-rate band is £125,000 for standard buyers, £300,000 for first-time buyers. Scotland uses LBTT and Wales uses LTT with separate rates.

How many 25kg bags of cement for 1m3 of concrete?

For a C25 concrete mix (1:1.5:3), you need approximately 320kg of cement per m³ — that is 13 bags of 25kg cement. For C20 (1:2:4) you need approximately 240kg (10 bags). These figures include a 10% overage allowance. For pours over 1.5m³, ready-mixed concrete is more cost-effective.

How much does a new build house cost per m2 in the UK?

In 2025, new build house costs range from £1,800–£2,200/m² in lower-cost UK regions to £2,500–£5,500/m² in inner London for standard to high specification. A typical 3-bed self-build in the South East costs £2,200–£3,200/m². These figures exclude land, architect fees, planning and VAT.

Free Build Cost Calculator, Stamp Duty & Construction Calculators UK 2025

Updated April 2025 — UK & London Rates

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7 free UK construction calculators — house build cost per m², stamp duty (SDLT), brick & block quantities and concrete mix. All rates updated for 2025. No sign-up. Instant results.

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Use our free house build cost calculator to estimate the cost per m² for extensions, loft conversions, refurbishments and new builds across London and the UK. Based on 2025 BCIS-aligned rates, our calculator adjusts for location, build quality and specification — giving you an accurate house extension cost estimate, loft conversion cost per m² London, or new build cost per m² UK in seconds. Select your project type below.

Location

Extension Type

Build Quality

Procurement

Conservation / Listed?

Floor Area

30 m²

5m²50m²100m²150m²200m²

New Kitchens

New Bathrooms

Estimated Build Cost

Select options

Fill in all fields for an instant estimate

2025 BCIS-aligned rates. Excludes VAT, architect & planning fees. Add 10–15% contingency. Always get a written quote.

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Location

Conversion Type

Build Quality

Listed Building?

Usable Floor Area

30 m²

10m²25m²40m²60m²80m²

New Bathrooms / En-Suites

Estimated Build Cost

Select options

Fill in all fields for an instant estimate

Includes structural, roofing, insulation, windows, plasterwork & basic fit-out. Excludes VAT and professional fees.

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Location

Refurb Extent

Finish Quality

Listed Building?

Total Floor Area

80 m²

20m²100m²200m²350m²500m²

New Kitchens

New Bathrooms

Estimated Cost

Select options

Fill in all fields for an instant estimate

Assumes typical 1970s–2000s UK residential. Structural, damp-proofing and asbestos removal costs are additional where required.

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Location

Build Type

Build Quality

Green Belt / Restricted?

Total GIA Floor Area

120 m²

40m²150m²300m²450m²600m²

Kitchens

Bathrooms

Estimated Build Cost

Select options

Fill in all fields for an instant estimate

Excludes land, planning, architect & engineer fees. Substructure varies by site conditions. VAT at 20% — self-build may qualify for zero-rating.

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Our free stamp duty calculator 2025 covers SDLT (England & Northern Ireland), LBTT (Scotland) and LTT (Wales) — updated for rates effective 1 April 2025. Calculate stamp duty for first-time buyers, home movers, buy-to-let and second homes (additional property surcharge), and limited company purchases. Enter your purchase price below for an instant banded breakdown.

Property Purchase Price (£)

Buyer Type

Property Type

Country

Rates shown are effective from 1 April 2025. The temporary SDLT nil-rate band increase ended 31 March 2025. Standard rates have reverted. Always confirm with your solicitor before exchange.

Stamp Duty / Land Tax

Enter price

Enter a purchase price to calculate

SDLT is payable within 14 days of completion. First-time buyer relief applies on purchases up to £500,000 (0% on first £300,000 from April 2025). Additional property surcharge is +5% on all bands.

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Our free brick calculator UK works out how many bricks or blocks you need for any wall. Standard half-brick walls use 60 bricks per m². Full brick walls need 120 bricks per m². Concrete blocks use 10 per m². Enter your wall dimensions and openings below — the calculator automatically deducts windows and doors and includes mortar quantities and a 5% wastage allowance.

Wall Length (m)

Wall Height (m)

Wall Type

No. of Windows (std 1.2×1.0m)

No. of Doors (std 0.9×2.1m)

Mortar Type

A standard house brick is 215×102.5×65mm with a 10mm mortar joint. Thermalite/Aircrete blocks are 440×215×100mm. All quantities include 5% wastage allowance.

Materials Required

Enter dimensions

Enter wall dimensions to calculate

Quantities include 5% wastage. Premixed mortar 1 tonne bag = approx 1,000 standard bricks. Order from same batch to ensure colour consistency. Always allow extra for cutting on corners and reveals.

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Our free concrete mix calculator tells you exactly how much cement, sharp sand and 20mm aggregate you need for any pour. Choose your concrete mix ratio — C10, C20, C25 or C30 — and get results in 25kg bags or bulk tonnes. Works out how much concrete you need for paths, driveways, slabs and foundations. All quantities include a 10% overage allowance.

Length (m)

Width (m)

Depth (mm)

Path 75mm · Slab 100mm · Foundation 300mm+

Concrete Grade / Mix Ratio

Output Unit

For pours over 1.5m³ we strongly recommend ordering ready-mixed concrete — it is more consistent and typically more cost-effective than site-mixing. Water/cement ratio should be 0.45–0.55 for structural work.

Materials Required

Enter dimensions

Enter dimensions to calculate quantities

Quantities include 10% overage. Always mix concrete in small batches to maintain consistency. Use a ratio of approx 0.5 litres of water per kg of cement. Do not add excess water — it weakens the mix significantly.

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2025 Cost Reference

UK Build Cost Per m² Guide — London & Regions 2025

Typical cost-per-square-metre ranges for common construction projects. London commands a 15–30% premium over UK averages. All figures exclude VAT, architect fees and planning costs.

🏠

House Extension Cost

£2,200 – £3,800/m²

Single storey rear extension London 2025. Inner London 15–25% above outer boroughs. Side return and wrap-around extensions from £2,750/m².

🏗️

Loft Conversion Cost

£1,500 – £3,200/m²

Dormer loft conversion from £45k. Mansard from £70k. L-shaped dormer from £60k. Listed building and conservation area premiums apply.

🔨

Refurbishment Cost

£600 – £1,800/m²

Minor cosmetic refurb from £600/m². Extensive full-strip refit up to £1,800/m². Average 3-bed full refurb: £50,000–£120,000 in London.

🏢

New Build Cost

£2,500 – £5,500/m²

New build cost per m² from £2,200 in lower-cost UK regions. Inner London high-spec builds exceed £5,500/m² for bespoke finishes.

Know Your Numbers

What's Included in These Estimates?

Understanding exactly what is and isn't covered ensures your project budget is accurate from day one.

✅ Included in Estimates

✓Foundations, groundworks & drainage
✓Structural frame, walls & roof structure
✓Windows, external doors & glazing
✓Insulation & airtightness measures
✓First & second fix carpentry and joinery
✓Plumbing & heating (standard specification)
✓Electrical first & second fix
✓Plastering, screeding & floor finishes
✓Decoration (2-coat paint throughout)
✓Mid-range kitchen & bathroom units (where selected)

❌ Excluded — Budget Separately

✗Land purchase & legal/conveyancing fees
✗Architect & planning consultant fees (£3,000–£15,000+)
✗Structural engineer fees (£1,000–£5,000)
✗Planning application & building regs fees
✗VAT at 20% on most construction works
✗Party wall surveyor costs where required
✗Bespoke fitted furniture & wardrobes
✗Landscaping, drives & external works
✗Utility connection & diversion costs
✗10–15% contingency buffer (always add this)

Common Questions

Construction Calculator FAQs

Answers to the most commonly searched questions about UK build costs, stamp duty, bricks, blocks and concrete mixes in 2025.

In 2025, a standard single storey rear extension costs £2,200–£3,800 per m² in London (excl. VAT). A 25m² rear extension in outer London costs approximately £55,000–£95,000; in inner London, £65,000–£115,000. Double storey extensions add 15–25% to the base m² rate. Side return and wrap-around extensions start from £2,750/m². All figures exclude architect fees, planning costs and VAT.

A rear extension in London costs £55,000–£115,000 for a typical 25m² single storey, depending on whether you're in outer or inner London and your specification level. At standard spec, outer London, expect around £65,000–£75,000 all-in. High-spec, inner London projects can reach £90,000–£115,000. These costs include structural, roof, windows, electrics, plumbing and plastering but exclude architect fees, planning and VAT.

Loft conversions in London typically cost £1,820–£2,380 per m² for a good-quality finish (excl. bathrooms). A standard 30m² dormer conversion in outer London costs approximately £55,000–£75,000. Mansard conversions in inner London range from £75,000–£130,000+. An en-suite adds £8,000–£14,000 to the budget. Use our loft conversion cost calculator above for a personalised estimate.

UK building costs per m² in 2025 vary significantly by project type and region. Typical ranges: Extensions: £1,800–£3,800/m² · Loft conversions: £1,500–£3,200/m² · Refurbishment: £600–£1,800/m² · New build houses: £1,800–£5,500/m². London commands a 15–30% premium over the UK average. All figures exclude VAT, professional fees and land costs.

A new build house costs £1,800–£2,500/m² for standard specification in lower-cost UK regions. In the South East and outer London, expect £2,200–£3,200/m². Inner London and high-specification builds range from £3,000–£5,500/m². A typical 120m² 3-bed self-build in outer London costs approximately £265,000–£385,000 (construction only, excl. land, design fees and VAT).

A side return extension typically costs £2,750–£3,400/m² in London due to tight access, party wall considerations and the need to tie into the existing side wall. A typical 15–20m² side return costs £45,000–£75,000 all-in depending on location and spec. Many Victorian terraces in West and South London use side return extensions to create open-plan kitchen-diners.

From 1 April 2025, a home mover buying a £300,000 property pays £3,750 in SDLT (0% on first £125,000 + 2% on next £125,000 = £2,500 + 5% on final £50,000 = £1,250). A first-time buyer pays £0 as the property is under the FTB £300,000 nil-rate threshold. A buyer of an additional/buy-to-let property pays £18,750 (standard duty + 5% surcharge on the full amount).

On a £400,000 purchase from April 2025: a home mover pays £10,000 SDLT (0% on £125k + 2% on £125k + 5% on £150k). A first-time buyer pays £5,000 (0% on £300k + 5% on £100k). For an additional/buy-to-let property, the buyer pays £30,000 (standard duty + 5% surcharge on the full £400,000 from £0).

From 1 April 2025, first-time buyers pay 0% on the first £300,000 and 5% on the portion from £300,001–£500,000. Properties over £500,000 receive no first-time buyer relief (standard rates apply). Example: £450,000 purchase = 0% on £300,000 + 5% on £150,000 = £7,500 total SDLT. FTB relief is not available if you (or a joint buyer) has previously owned property anywhere in the world.

From October 2024 (and remaining effective in 2025), the additional dwelling surcharge is 5% on top of standard SDLT rates, applied from £0 on the full purchase price. So a £300,000 buy-to-let property attracts: 5% on the full £300,000 = £15,000 surcharge + standard duty of £3,750 = £18,750 total SDLT. Use our stamp duty calculator above to get an exact breakdown for your purchase price.

SDLT (Stamp Duty Land Tax) is a tax paid by property buyers in England and Northern Ireland. It is calculated on a tiered/banded basis — each band of the purchase price is taxed at a different rate. From April 2025: 0% on first £125,000 / 2% on £125,001–£250,000 / 5% on £250,001–£925,000 / 10% on £925,001–£1.5m / 12% above £1.5m. Scotland uses LBTT and Wales uses LTT with separate rate tables.

A standard UK brick is 215×102.5×65mm. With a standard 10mm mortar joint: a half-brick wall (102.5mm) uses 60 bricks per m². A full-brick wall (215mm) uses 120 bricks per m². Lightweight concrete blocks (440×215×100mm) use 10 blocks per m². Always add at least 5% for wastage, cuts and breakages. Order from the same batch to ensure colour consistency.

For a standard half-brick wall, you need approximately 2.5 bags of premixed mortar (25kg) per m². For a full-brick wall, allow 5 bags/m². For 100mm concrete blocks, allow 1.2 bags/m². One 25kg bag of premixed mortar typically lays around 25–30 standard bricks. For large projects, buying bulk sand and cement separately is more cost-effective — a 1-tonne bag of premixed mortar lays approximately 1,000 standard bricks.

A standard UK brick is 215mm long × 102.5mm wide × 65mm tall (BS EN 771-1). With a 10mm mortar joint, the working dimensions are 225×112.5×75mm. The most common brick bond in the UK is stretcher bond (used in half-brick cavity walls). Engineering bricks and wirecut bricks are the same size. Handmade or reclaimed bricks may vary slightly — always allow extra wastage.

For a garden path or patio: use C20 mix (1:2:4) at 75–100mm depth. For a driveway with car loading: use C25 (1:1.5:3) at 100–150mm. For heavy vehicle loading or HGV use: C30 (1:1:2) at 150mm minimum. For house foundations (strip or pad): C25 or C30 at the depth specified by your structural engineer. Do not use C10 lean mix for any load-bearing application.

The number of cement bags per m³ depends on the mix: C10 (1:3:6): ~7 bags · C20 (1:2:4): ~10 bags · C25 (1:1.5:3): ~13 bags · C30 (1:1:2): ~17 bags. These are 25kg bags. For a typical 100mm deep, 4×3m garden slab (1.32m³ inc. 10% overage), you need approximately 17 bags of cement for C25. Use our concrete calculator above for your specific dimensions.

C20 (1:2:4 ratio, 20N/mm² strength) is suitable for general domestic purposes — paths, patios, steps and light driveways. C25 (1:1.5:3 ratio, 25N/mm²) is the standard for house foundations, floor slabs and structural footings — it's the most commonly used mix in UK residential construction. C30 (1:1:2 ratio, 30N/mm²) is used for heavily reinforced work, retaining walls, and driveways with regular HGV use.

Order ready-mixed concrete for any pour over 1.5m³ — it's more consistent, faster and usually more cost-effective at scale. Ready-mix is also preferable where access allows and for time-sensitive pours (hot weather, large slabs). For small DIY jobs under 1m³ (e.g. fence posts, small slabs), bagged concrete or site-mixing with a mixer is practical. In London, ready-mixed concrete typically costs £110–£180/m³ depending on grade and supplier lead time.

More Information

How to Use These Calculators

🏠 Build Cost Calculator

Our house build cost calculator uses 2025 BCIS-aligned cost-per-m² rates adjusted for your location, quality and project type. It covers rear extension costs, loft conversion costs per m², full house refurbishment and new build cost estimates. Results are typically accurate to ±15–20% — use as a feasibility guide before getting written quotes.

💷 Stamp Duty Calculator 2025

Our SDLT calculator is updated for the April 2025 stamp duty changes — the nil-rate band has reverted to £125,000 for standard buyers and £300,000 for first-time buyers. We cover all buyer types: first-time buyer stamp duty, home movers, buy-to-let stamp duty, additional properties and limited company purchases in England, Scotland (LBTT) and Wales (LTT).

🧱 Brick & Concrete Calculators

Our brick calculator tells you how many bricks per m² you need — 60 for half-brick walls, 120 for full-brick, 10 for lightweight blocks. Our concrete calculator works out cement, sand and aggregate for C10 to C30 mixes. Both tools include wastage allowances and can output in bags or bulk tonnes — ideal for DIY and trade use.

About These Calculators — UK & London 2025

All construction cost calculators on this page are maintained by Signature Build Properties, a London-based design and build contractor with 15+ years experience across all London boroughs. Rates are reviewed quarterly against BCIS data, Federation of Master Builders surveys and live tender returns.

Our extension cost calculator London uses base rates of £2,200–£3,800/m² with location multipliers for inner London (×1.30), outer London (×1.15) and UK regions down to ×0.78 for the North East. Our stamp duty calculator reflects the April 2025 changes in full, including the reverted nil-rate bands and the 5% additional dwelling surcharge introduced in October 2024.

For material calculators: our brick calculator is based on BS EN 771-1 standard brick dimensions (215×102.5×65mm) and our concrete mix calculator uses BS EN 206 mix design ratios aligned with current UK practice. All material quantities include standard wastage allowances used by professional quantity surveyors.

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UK Build Cost Per m² Guide — London & Regions 2025

House Extension Cost

Loft Conversion Cost

Refurbishment Cost

New Build Cost

What's Included in These Estimates?

✅ Included in Estimates

❌ Excluded — Budget Separately

Construction Calculator FAQs

How to Use These Calculators

🏠 Build Cost Calculator

💷 Stamp Duty Calculator 2025

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About These Calculators — UK & London 2025

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UK Build Cost Per m² Guide — London & Regions 2025

House Extension Cost

Loft Conversion Cost

Refurbishment Cost

New Build Cost

What's Included in These Estimates?

✅ Included in Estimates

❌ Excluded — Budget Separately

Construction Calculator FAQs

How to Use These Calculators

🏠 Build Cost Calculator

💷 Stamp Duty Calculator 2025

🧱 Brick & Concrete Calculators

About These Calculators — UK & London 2025

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