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Costs of extension

I'm trying to get a ball park figure of what a side return extension measuring 4.5 x 1.5 metres in London should cost. The general consensus is that it's around £1200 per metre squared, which to me would add up to £8100. Wherever I've looked I've seen costs of extensions to be from £30000 to £110000 for a side extension so I'm really confused. Does the cost per metre squared apply to the new size of the room? What am I missing?

I would also love to know what proportion of the costs is labour costs and what proportion is materials.

I'm having drawings done by an architect as we speak but with the astronomical figures I'm seeing I'm considering giving up on the idea.

Thanks so much for any information.

2 Answers from MyBuilder Extension Builders

Best Answer


Here in Glasgow a single storey extension is generally priced at between £1000 to £1200 / m2
I agree with you on a figure of £8100 to erect the shell and make it wind and watertight Electrical work included in that price and even plumbing if reqd.
Only a builder can explain the breakdown but the main cost is the labour itself.
Hope this helps


Answered 6th Feb 2018

The problem you have is that it is a very small extension so there are no economies of scale. I built a similar sized side extension on on my own house, this came in at £10,600 fully fitted, planning costs, drawings etc. but with minimal labour costs. The usual split is roughly 50:50 in London.


Answered 25th Mar 2015

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