I have been estimating build costs for different sized houses of identical 2 storey “standard” specification on the same plot in Leicester (East Midlands). The build cost calculator indicates that build costs per square metre increase as the size of the house increases.

Small house (less than 113 sq m GIA) = £723 psm
Medium (113 – 140 sq m) = £783 psm
Large (over 140 sq m) = £929 psm

A similar result is shown when I double checked this against building in the South East.

I was expecting economies of scale to apply resulting in a greater cost per sq m for very small houses and a significantly reduced cost per sq m for larger houses (all other things remaining equal).

Have I made a mistake somewhere?

  • Adam

    You may have made a mistake James but then again you may not have.

    It is difficult to plot build costs onto a linear graph that shows decreasing costs as the square meterage increases. You are basing your cost assumption on increased value as the size also increases. This is true to a point. At some point the value gained from purchasing materials in bulk plateaus. In order to get the next level of discount you would have be buying vast quantities.

    Above certain sizes some materials become exponentially more expensive. For example greater span sizes across rooms mean bigger and sturdier bits of timber, engineered truss or steel to support the floor/roof above. These things do not get cheaper the bigger they get. In fact there are ‘best value’ sizes, where the price for the most common sizes, are cheapest. These tend to be the sizes you’d find on a standard 3 bed semi. Above that (eg 4.8m lengths of timber) you move into the realms of specialist suppliers or bespoke cutting/fabrication.

    I suspect that this is what you have experienced. There are some very good build cost estimation programmes available these days for the self builder. If you think you’ll be doing this again you could invest. If not, you’ll find your local builders merchants offer a free cost estimating service. If you take them a copy of your structural plans they will draft you a complete costing for the building of the house. There are limits to their abilities and they can’t factor in every cost to the nth degree but for the fabric of the building they can be pretty good. And of course it helps to play one off against another. You’d be surprised where they "discover" opportunities for a discount when faced with the prospect of losing 60-120k of business to a competitor…..

    Good luck whatever you do.

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