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:

• Kitchen supply and fitting
• Bathroom supply and fitting
• Decoration
• Floor finishes
• External works

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

• Single storey rear extension, replacing an existing Kitchen offshoot, which was completely removed
• Some internal re-working, including the creation of an internal shower room.
• The glass doors to the rear were a reasonably high spec, and the roof was covered in a sedum green roof.
• Extension Floor Area= 27.66m2
• Internal Refurbished Floor Area= 18.93m²
• Total Project Cost= £80,900+VAT
• Project Cost minus variable aspects= £65,655+VAT
• Area based price (extension)= £1,630+VAT/m²
• Area based price (refurbished)= £1,087+VAT/m²

Case Study 2

• Single storey rear extension, connecting to existing extension
• Significant internal re-working, with multiple levels
• The sliding glass doors to the rear were a very high spec, as was the window. The overhanging roof would also have added to the costs. There were serious access restrictions.
• Extension Floor Area= 10.56m2
• Internal Refurbished Floor Area= 32.63m²
• Total Project Cost= £141,907+VAT
• Project Cost minus variable aspects= £78,410+VAT
• Area based price (extension)= £2,427+VAT/m²
• Area based price (refurbished)= £1,618+VAT/m²

Case Study 3

• Single storey rear extension, with 2 storey section containing first floor bathroom. Loft conversion with dormer window
• Whole house refurbishment
• Generally a fairly standard spec throughout
• Extension Floor Area= 29.85m2
• Internal Refurbished Floor Area= 58.62m²
• Total Project Cost= £140,162+VAT
• Project Cost minus variable aspects= £130,337+VAT
• Area based price (extension)= £1,891+VAT/m²
• Area based price (refurbished)= £1,261+VAT/m²

Case Study 4

• Single storey rear extension, loft conversion with dormer window. Stand alone Garden Office
• Whole House Refurbishment
• Generally a fairly standard spec throughout. The Client undertook the cladding of the garden office themselves to help control costs.
• Extension Floor Area= 51.94m2
• Internal Refurbished Floor Area= 72.74m²
• Total Project Cost= £115,725+VAT
• Project Cost minus variable aspects= £93,429+VAT
• Area based price (extension)= £930+VAT/m²
• Area based price (refurbished)= £620+VAT/m²

Case Study 5

• Single storey rear extension, replacing existing galley Kitchen. Loft conversion with dormer window
• Some internal re-working to top floor plus first floor windows
• Generally a fairly standard spec throughout. Wildflower green roof on extension
• Extension Floor Area= 26.45m2
• Internal Refurbished Floor Area= 20.00m²
• Total Project Cost= £82,966+VAT
• Project Cost minus variable aspects= £68,818+VAT
• Area based price (extension)= £1,730+VAT/m²
• Area based price (refurbished)= £1,153+VAT/m²

Case Study 6

• Single storey rear extension, replacing existing conservatory, with two storey section housing bedroom.
• Whole house refurbishment, including rendering of house and replacement of all windows. Price includes for installation of ground source heat pumps and piping
• Generally a fairly standard spec throughout
• Extension Floor Area= 36.85m2
• Internal Refurbished Floor Area= 107.07m²
• Total Project Cost= £185,830+VAT
• Project Cost minus variable aspects= £158,824+VAT
• Area based price (extension)= £1,467+VAT/m²
• Area based price (refurbished)= £978+VAT/m²

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]