How much is it?

For extension cost there is a basic rule of thumb and we can work it out with a rough budget on £ per m². A decade ago this was quite simple math at £1000 per m² but general build costs have risen, and it is more advisable to start from £1350 per m². So, for example, if you are building a 3m by 5m single storey extension (very common extension) this would come out at £20,250 (3 x 5 x 1350).

To add another floor to the same footprint as aforementioned single storey extension you would add an additional 50% of the build cost, so that’s adding £10,125 to the £20,250 to make £30,375. In addition you need to add VAT on top of it.


The main reason for extension cost increase was inflation which went up by 22% between 2008 and 2018. On top of it we had many changes to building regulations between 2006 and 2011 mainly involving higher requirements for insulation, foundation and roof structure.

Bear in mind that all above figures are for block shell only; without plumbing, electric work, decoration or flooring. Moreover, examples of prices are for London and surrounding areas.

Unpredictable Cost

Even the best builders in the land are not fortune tellers and finding unexpected additional costs once works have commenced is a regular occurrence on building projects.

Most building companies including us do their best to avoid unforeseen work. From experience we know what is a chance of a surprise. However, there is no way to tell what we are going to find during excavation process or if your foundation need extra reinforcement. Sometimes we can find even your neighbour’s pipe that will need to be rerouted or deep layers of clay which forces us to cast foundation even up to 2 m instead of 1m.

Nobody likes unplanned cost nor work. It disrupts our project schedule and drain your pocket. We do have, however policy where we don’t charge any profit or overhead for such type of work. Simply speaking you pay only the cost we must cover so at least we can minimise the burden on one side.