House extension materials planner
Choose a starting template, enter external dimensions and adjust walls, roof, windows and doors. Your 3D model and materials list update together. Free template-based estimating; no AI credits required.
- External footprint
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- Approx. internal floor
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- Net masonry face area
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- Roof area
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- Footing + slab concrete
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A planning take-off for the selected shell, not a complete bill of quantities. Check the assumptions and exclusions below.
View full materials list
| Material | Order allowance |
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Tap the extension to place your note
Grey back wall = existing house, excluded from the take-off. Blue panels = window openings; doors are open gaps. Foundations and roof timbers are not drawn. Wall assemblies are shown solid for clarity. Notes stay in this browser and are included in copied results.
How this estimate works
The footprint is external width × projection. The new wall run is width + twice the projection; the attached house wall is excluded. Wall area includes the front gable or lean-to side triangles, then deducts every window and door. The same approximate elevation area is used for each masonry leaf and wall lining, so corner, reveal and insulation details need checking.
Internal floor area deducts two side-wall thicknesses from the width and one front-wall thickness from the projection. Cavity wall thickness is the chosen leaves plus cavity width. The floor slab, hardcore and floor insulation use that internal area. Roofing uses the sloped area including overhang at exposed edges; no overhang is added at the house connection.
Material allowances you can change
Standard 215 × 102.5 × 65 mm bricks use about 60 bricks per m² with 10 mm joints; 440 × 215 mm block faces use about 10 per m². Coverage inputs can be changed for your product. See the merchant brick guide. The selected waste percentage is added to material quantities, except excavation and gutter length. Membranes get a separate 15% laps/edges allowance before waste.
Floor and wall insulation are area allowances, not thickness or performance specifications. Plasterboard assumes 1200 × 2400 mm sheets on approximate inner wall faces and a flat ceiling. Flat-roof deck sheets are 2440 × 1220 mm; board thickness and supports are not designed. Pitched roof coverage and batten gauges are indicative, as explained in the roofing calculator.
Worked example
The default 5 × 3 m rear extension has a 15 m² footprint, three new walls totalling 11 m, and 3.69 m² of window and door openings. At 2.4 m eaves height, net masonry face area is 22.71 m². With 10% waste, a brick-and-block cavity wall allows 1,499 facing bricks and 250 blocks. These are above-ground quantities only.
What is not included
- Demolition and openings through the existing house, steel beams, lintels, reinforcement, structural timber/trusses, fixings and wall ties.
- Below-ground masonry, foundation engineering, soil suitability, floor excavation, spoil bulking and disposal.
- Drainage, flashings, rooflights, downpipes, electrical work, plumbing, heating, flooring, paint and fitted doors/windows.
- Mortar ingredients, plaster/render bag yields, insulation thicknesses, cavity barriers, fire stopping, DPC/DPM junctions and product-specific detailing.
This is an early material allowance, not a structural or Building Regulations design. A single block wall or any template is not a statement that an occupied extension will meet thermal, moisture or structural requirements. Have foundations, wall construction, openings, roof structure and regulatory requirements checked by your designer before ordering or building.
Questions
Does the tool include the existing house wall?
No. It assumes the extension is attached along its back edge and counts three new external walls. Alterations and structural openings through the house are excluded.
Can I change the windows and doors?
Yes. Add up to eight openings, choose their wall, size, sill height and offset. Their areas are deducted and the openings appear in the model.
Is this AI-generated or does it use API credits?
It is a template-based calculation that runs in your browser. It does not send designs to an AI service or use API credits.
Are the materials ready to order?
Use them as an early allowance. Confirm product coverage, construction details, quantities and structural specifications with your designer and supplier first.
Results are estimates for planning and ordering. Check quantities with your supplier and product data sheets, and anything structural or regulated with a qualified professional. See our terms.