Free tool

Extension material estimator

Foundations to roof — full material list in seconds. Plus a U-value checker to confirm you meet Part L. For a proper takeoff from your drawings, get in touch.

Prices last updated: July 2026 · Indicative trade rates ex VAT

Location

Adjusts all prices for regional merchant rates and delivery.

Dimensions

Doors & windows total area

Foundations

Walls

Ground floor

Roof structure

For sizing and spec, see velux.co.uk/products/roof-windows ↗

Ceiling & lintels

Count each door and window opening

Underfloor ventilation

Typically 1 per 1.5m of wall, min 2 per wall

Flexible pipe connecting air bricks across the sub-floor void

Plastic inserts in mortar joints above DPC — approx every 900mm

Material list

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Adjust inputs to see your material list.

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Browse popular UK bricks and blocks from leading manufacturers. Click through to spec the right product for your job — prices in the estimator update to match the type you select.

Browse popular UK bricks and blocks to help spec your job. Prices in the estimator are based on standard trade rates — speak to your merchant for exact product pricing.

Outer leaf

Cavity

Inner leaf

Calculated using BS EN ISO 6946 simplified method (sum of thermal resistances). Part L 2021 requires ≤ 0.18 W/m²K for new extension walls in England.

U-value result

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W/m²K
Select options above

Passing combos at 100mm cavity

All meet Part L with a 100mm cavity — saving money on wider lintels, blocks and foundations vs 150mm.