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.

📋 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.

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⚠ Exclusions
  • Demolitions & alterations to the existing dwelling are assessed separately using the slider below — adjust to reflect your project scope.
  • External landscaping, boundary works, driveways and drainage can be included via the External Works slider. If set to None, external works are excluded from this estimate.
🏠 BuildIQ — New Build Dwelling Calculator is under development and due Late 2026. Register your interest at mail@kevinbarryqs.com.
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Floor Area
Size 30 m²
5 m²30 m²100 m²
Demolitions & Alterations to Existing Dwelling
None Included in base rate
NoneMinorMod.Signif.Extens.W-H

No alterations to existing dwelling beyond the connection — access opening already priced in Access options above

Works to the existing dwelling arising from the extension project — over and above the connection opening already priced in Access options above. Select the level that best reflects the extent of works required to the existing home.

Siteworks BCIS 6A
None Not included
NoneMin.MinorMod.Signif.Extens.

No siteworks — grounds unaffected by the extension works.

Footpaths, hardstandings, paving, boundary walls & fencing, regrading and garden making-good arising from the extension.

Drainage BCIS 6B
None Not included
NoneMin.MinorMod.Signif.Extens.

No drainage works — surface water and foul connections unaffected.

Surface water disposal, foul connections, manholes, soakaways and drain diversions arising from the extension.

Specification Quality
Good Level 3 of 5
Basic Std. Good Prem. Luxury

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
🪟
Rooflights (Flat / Flush)
+£1,500–£3,000 each
0
🔆
Roof Lanterns (Structural)
+£4,500–£9,000 each
0
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
Estimated Cost Band — Free Preview
Your project falls in this cost band
Lower cost Higher cost
excl. VAT · band updates live as you adjust
— — — 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 dwelling assessed via slider — set level below
✗Furniture, appliances, and decoration are excluded
✗Listed building or conservation area uplift is not modelled
✗Abnormal ground conditions (rock, water table) are excluded
Your estimate is ready
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Cost Heatmap — All Permutations
Mid-point build cost across every spec × size combination — blue lowest, red highest
Lowest cost
Highest cost
Hover / tap any cell for exact mid-point figure. Cost range shown in parentheses (low – high). Excl. add-ons & siteworks unless shown.
Cost Curve — Cheapest Configuration
Basic spec · single storey · flat roof · no add-ons · no siteworks
Unlock to view accurate figures
Low estimate
Mid estimate
High estimate
Excl. VAT · no add-ons
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