Cost of Domestic Extensions

NOTE: This article was originally written in 2020, including projects completed prior to this. See note at bottom of page.

One subject that comes up on every project is, of course, cost. We all only have a certain amount of money, and want a clear idea of how far our budget can go. It is always a hard one to tackle, as every project is different, and every client has differing needs and levels of specification. We are not cost consultants, and the best way to get an accurate price is to speak with a number of builders. That said, what we can do is look at past projects and see what price they came to.

For this piece, we have chosen to look at a number of examples from the past few years of extensions we have completed in and around York. Each of these have an element of extension and some refurbishment of the existing house. To make for a fair comparison, we have discounted some of the more variable aspects of work, namely:

We have then made an assumption that the costs of the refurbishing elements of the house are at a rate of rds that of the extension. We have assumed that loft conversions, needing new floor construction, dormer windows, insulation etc are at the same rate as extensions. It is a fairly crude tool, but the best approach without going into full detail, and useful as a guide.


Case Study 1

Case Study 2

Case Study 3

Case Study 4

Case Study 5

Case Study 6

As can be seen from the figures, there is some variation, which reflects a number of things, including complexity of project and also economies of scale. Looking at the full house refurbishments, they generally are able to achieve a better overall rate compared to smaller extensions which have more foundations, insulation, structure per floor area than the larger projects. Generally, we are finding the rates for extensions are in the region of £1,400-£1,900/m2 with refurbished areas at £950-£1,300/m2. There are some outliers where an extremely cheap or expensive rate is achieved, but these are the exceptions. There may be several reasons why these don’t fit in the normal pricing banding. The expensive one was a high spec job, in particular with the glazing. There were a lot of external works included within the overall contract sum, and while I have discounted these, it is entirely likely the contractor made allowance for these works within the other prices and then post-rationalised the external works price out. Again, I would like to stress that there is no real science in the above, and floor area based estimations are a pretty blunt tool. When considering an area based estimation, don’t forget to add in the VAT, or to allow for all of the aspects we have discounted.

NOTE: This article was originally written in 2020 with projects completed prior to then. Since then, there have been significant financial pressures on construction so for domestic extensions we are generally advising that a square metre rate of upwards of £2500+VAT/m2 should be used as a starting point. [October 2022]