What Materials Do You Need for a House Extension (Cost Breakdown)

Planning a house extension is exciting. Then you start pricing up materials and realise you have no idea where to begin.

The list is longer than most people expect, and getting it wrong costs more than getting it right the first time. Here's what you actually need, what it costs, and where it pays to spend properly.

The Main Material Categories

Blocks and Bricks

Blocks form the inner leaf of a cavity wall (dense aggregate or lightweight, typically 100 mm or 140 mm). Bricks are the structural backbone of any masonry extension, and go on the outer leaf, and matching them to your existing property takes more effort than people expect. An older house with a discontinued brick type can add real cost to this category.

Typical cost share: 15 to 20% of total material spend

Timber

Used throughout: floor joists, wall plates, rafters, studwork for internal partitions. Structural timber needs to be the right grade (C16 or C24) and properly dried. Wet or undersized timber causes problems later: squeaking floors, warped frames, movement around window openings. Engineered timber (I-joists, LVL beams) comes in where longer spans are needed.

Typical cost share: 10 to 15% of total material spend

Insulation

Building regulations set a floor, not a target. Walls, floors, and roofs all need to be insulated properly: rigid PIR board in the cavity, 100mm under the screed, and the right depth between rafters or at ceiling level, depending on roof type. A couple of hundred pounds saved on thinner board adds to your heating bill for the life of the building.

Typical cost share: 8 to 12% of total material spend

Roofing

The roof is your primary defence against water ingress, and repairs are expensive. For flat roofs, GRP fibreglass or EPDM rubber membrane are the standard choices now, both warrantied for 20 to 25 years when fitted correctly. Pitched roofs need concrete or clay tiles (or slate), breathable felt, battens, and counter-battens. Flashings, drip edges, and eaves ventilation all matter too.

Typical cost share: 12 to 18% of total material spend

Fixings, DPC, and Sundries

The category that holds everything together and gets underestimated on almost every materials schedule. Wall ties, joist hangers, structural brackets, DPC and DPM, cavity closers, lintels: none of these are areas to value-engineer. Wrong tie spacing, missing DPC, or inadequate lintels are building regulations issues that show up at inspection or when you come to sell.

Typical cost share: 5 to 8% of total material spend

Cost Breakdown by Category

Material costs only. Excludes labour, groundworks, VAT, and architect and structural engineer fees.

Estimated Material Cost for a 20m² Extension

This would be around £8,000 to £15,000 in materials, depending on the specification.

The lower end is a straightforward flat-roof lean-to with standard brick and block and UPVC windows. The upper end reflects a pitched roof, matching facing brickwork, aluminium glazing, and higher-spec insulation. Overall build cost for a 20m² extension (including labour and groundworks) typically sits between £24,000 and £40,000 at current UK rates.

Why Cutting Corners Costs More

The savings on cheaper materials rarely hold up. Inferior roofing membrane fails early, under-spec insulation costs more in energy bills every year, and then wrong fixings create compliance issues. When remedial work is needed, it costs more to fix than the original saving was worth, on top of the disruption of going back to a finished job.

For builders especially, the materials you sign off on reflect on your workmanship. Good materials, sourced consistently and on site when you need them, make the whole job run better.

Buy Your Greater Manchester Extension Materials at Joseph Parr Alco

We supply the full material range for house extensions: block, brick, timber, insulation, roofing, fixings and more, delivered direct to site across the region.

We work with builders on full material packages, so you're not chasing multiple suppliers for one job. Consistent stock, reliable delivery, and trade accounts for both one-off builds and ongoing work.

Pricing up an extension? Get in touch and we'll help you get the specification right.