Step Two

Having found the appropriate rate (£/m2) from the H&R Average Build Cost Guide, make any adjustment for quality or unusual materials, such as stone, natural slate, thatch etc. that may be necessary (see below) and then multiply the adjusted £/m2 by the gross internal floor area of your design in m2 to calculate a build cost estimate.

Calculate any Adjustments

Using Roofspace: Using roofspace to provide accommodation is less expensive than adding a full extra storey. For estimating purposes, calculate the cost separately by multiplying the additional useable gross floor area by 70% of the average £/m2 for the house.

Basements: The cost of constructing a basement varies according to ground conditions. For estimating purposes, treat basements as above ground space and include them as part of your measurement of gross internal floor area.

External Facing Materials: Alternative facing materials, e.g. handmade bricks, natural stone, flint panels, timber or render will have a direct influence on overall build costs. To adjust for this you need to add or subtract an allowance/m2 for your chosen wall cladding from the sum allowed for cladding within your chosen specification. Eg. for render on blockwork allow £25/m2. For timber cladding allow £25/m2. For handmade weathertiling allow £54/m2. For plain clay weathertiling allow £32/m2. For rubble walling/flint allow £90/m2. For reconstituted stone allow £48/m2. For natural stone allow £75/m2. For Ashlar allow £130/m2.

Roofing Materials: Alternative roofing materials, e.g. slate, stone, handmade clay tiles and thatch will have a direct effect on your overall build costs. To adjust for this, add or subtract an allowance/m2 for your chosen roofing material from the figure allowed for roofing in your chosen Level of Build Quality (E.g. allow £45/m2 for plain clay tiles. For new Welsh Slate allow £60/m2. For second-hand slates allow £35/m2. For handmade plain clay tiles allow £65/m2. For reed thatch allow £80/m2.

Specialist Foundations: The build costs indicated in the Average Build Cost Guide are for conventional concrete foundations of up to 0.5m depth. The cost of developing a site with unusual ground conditions, e.g. clay, can add considerably to your build costs. Always ask your engineer or designer to investigate the ground conditions at the earliest possible stage.

Sloping Sites: For estimating purposes, allow an additional £10/m2 for every degree of slope, e.g. for a 5° slope, add approximately £50/m2.

Bespoke Joinery: To adjust for bespoke handmade windows and doors, add the joinery quote (excluding VAT) to your estimated build cost, less the allowance already in the costs of £40/m2.

Bespoke Kitchen: To adjust for a bespoke kitchen in a good or standard quality spec. house add the quote for your kitchen (excl. VAT) to your estimated build cost, less an allowance of £20/m2 for a standard or good specification or £40/m2 for excellent.

Ceiling Heights: For every 0.1m of additional ceiling height (on top of the standard 2.4m) add 1% to your £/m2.

Garages & Outbuildings: Although an integral garage is unlikely to be completed to the same standard of finish as the rest of the property, for cost estimation purposes include the area of integral garages within your measurements of the gross internal living area.