2026 Market Data www.quintinqs.com

Quintin QS · Professional Tool

UK Extension Cost Calculator

Get an instant, professional-grade cost estimate for your house extension — single or double storey — based on 2026 UK market data.

🚀 Soft Launch — This Month Only

ExtendIQ is launching this month at an introductory rate of £10 for the full PDF cost report. This price will increase next month. Early adopters lock in the best rate.

📋 Before engaging an Architect
Clients should have a realistic understanding of construction costs before commissioning design services. This report gives you that foundation — so you approach your Architect informed, with confidence and no costly surprises.

Your details are used only for enquiry follow-up. Never shared.

⚠ Exclusions General demolitions and alterations are excluded from this estimate. New extension works only.
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Floor Area
Size 30 m²
5 m² (micro)30 m² (typical)100 m² (large)
Specification Quality

Number of Storeys

GIFA is the total across both floors (e.g. 50m² = 25m² ground + 25m² upper). One set of foundations & roof serves both floors — double storey is typically 12% more cost-effective per m² of usable space.

Roof Type

Flat (Trocal warm roof) is standard for modern single-storey extensions. Pitched concrete tile costs more but may suit planning requirements or match the existing dwelling.

External Wall Finish

Painted render is standard for modern extensions. Facing brick matches many existing dwellings and adds durability. Western red cedar is a premium finish requiring periodic treatment.

Foundation Type

Piled foundations required on clay/shrinkable soil or near trees. Common across Midlands, London & South East.

Region
Add-Ons
Doors & Openings
🚪
Access from Existing
Internal opening to extension
🚪
Bi-Fold Doors (4-panel)
Exterior · +£5,000–£7,500
🚪
Sliding Doors (Exterior)
+£3,500–£7,000
Glazing & Light
☀️
Skylight / Roof Lantern
+£2,500–£5,000
Internal Fit-Out
🍳
Kitchen Integration
+£12,000–£25,000
🌡️
Underfloor Heating
+£2,000–£3,500
🚿
WC / Cloakroom
+£3,500–£5,500
Professional & Legal
📋
Planning & Architect Fees
+£3,000–£7,000
🏠
Party Wall Notice
+£1,500–£3,500 (surveyor fees)
Include VAT (20%)
Currently excluding VAT
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Quintin QS — professional feasibility and cost planning for NI & GB.
Estimated Cost Band — Free Preview
Your project falls in this cost band
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excl. VAT · band updates live as you adjust
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— — — Rate/m² 🔒 ⏱ — wks build
Industry Benchmarked
Rates stress-tested against RICS guidance, FMB 2026 surveys & real UK cost plans — calibrated for accuracy across all regions.

Build programme assumes a single experienced main contractor. Excludes design, planning approval and material procurement lead times. Allow 4–6 weeks additional contingency for typical projects.

Regional multipliers reflect 2026 UK construction labour and materials markets. Source: Quintin QS market analysis.

✓Single-storey rear extension on a standard residential dwelling
✓Concrete strip foundations on normal ground conditions
✓Flat warm roof (Trocal single-ply membrane) — base rate
✓External walls — painted sand/cement render on blockwork (base rate)
✓Standard cavity wall masonry construction with insulation
✓Basic electrical first fix — sockets, lighting, consumer unit connection
✓Plastered and painted internal walls and ceiling
✓Standard glazed access door included in shell cost
✓Rates based on 2026 UK market data — Quintin QS analysis
✗Demolitions/alterations to existing structure are excluded
✗Furniture, appliances, and decoration are excluded
✗Listed building or conservation area uplift is not modelled
✗Abnormal ground conditions (rock, water table) are excluded
🔒
Unlock Your Full Cost Report
Your indicative range is shown above. Pay £10 to unlock the full breakdown, build programme, market rates and a branded PDF report.
✓ Full trade-by-trade cost breakdown
✓ Mid-point estimate & rate per m²
✓ Build programme timeline
✓ Regional market rate comparison
✓ Branded Quintin QS PDF report

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