ExtendIQ — UK Extension Cost Calculator

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How It Works

60-second walkthrough — see how the calculator works.

Floor Area

Size Core build£—/m² 30 m²

5 m²30 m²200 m²

Economies of scale: the core build £/m² rate reduces as fixed standalone costs are spread over a larger area. It stabilises at 65 m²; above this, cost increases broadly in line with floor area. It excludes editable allowances and VAT.

Detached Outbuilding Benchmark

Source GIFA18 m²

Rebased reference£42,875

On-site programme14 weeks

High-spec standalone benchmark, including a full 360° envelope. Adjust the size slider for a modelled indication from 5–100 m²; the Northern Ireland source remains the 18 m² anchor and is rebased through the selected regional index.

Include benchmark site alterations, fence and path

Site alteration allowance for this scheme

Mechanical and plumbing are excluded in the reference. Fittings are unquantified. VAT, professional fees, statutory charges and upgrades are not included in the source benchmark.

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 NRM 1 8.2

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 NRM 1 8.6

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

🔆

Roof Lanterns (Structural)

+£4,500–£9,000 each

Internal Fit-Out

Specialist Works & Allowances

PS1

Existing heating & plumbing adaptations

Existing dwelling scope · NI reference £3,000

PS2

Existing electrical adaptations

Existing dwelling scope · NI reference £4,000

PS5

Make-good at existing roof abutment

NI reference £750

UPS1

Abnormal foundations / unforeseen ground

NI reference £500

UPS2

Incoming utility upgrades / diversions

NI reference £750

PS3

Kitchen / Utility fit-out

Replaces former kitchen toggle · NI reference £12,000

PS4

Shower / WC fit-out

Replaces former shower/WC toggle · NI reference £2,000

PS1

BWIC / services ducting

Source provisional sum · NI reference £324

PS2

Electrical & alarm installation

Source provisional sum · NI reference £2,160

PS3

Specialist fittings / deferred scope

Not included in source · enter only if instructed

These are client-selected allowances, not fixed calculator rates. They are included at the entered value before the standard 10% preliminaries, 5% contingency and any selected VAT.

Check for overlap: do not include the same patio, driveway or drainage works here if they are already selected in Demolitions & Alterations, Siteworks or Drainage. Use one allowance route only.

🌡️

Underfloor Heating

+£2,000–£3,500

🔥

Addition of Multi-Fuel Stove

+£2,400–£3,800

Professional & Legal

📋

Planning & Architect Fees

+£3,000–£7,000

🏠

Party Wall Notice

+£1,500–£3,500 (surveyor fees)

Include VAT (20%)

Currently excluding VAT

Secure payment via Stripe · Instant PDF on return

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 in the calculator inputs above

✗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 — Your Region

Mid-point build cost across every spec × size combination for your selected region — blue lowest, red highest

Region National Average

Storeys

Roof

Wall

Foundation

VAT

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

Region: National Average

Unlock to view accurate figures

Low estimate

Mid estimate

High estimate

Excl. VAT · no add-ons

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