House Extension Costs

Review of costs to build an extension

How much will my house extension cost?

This is a key question and should be investigated well before the house extension begins. If you are reading this, you may already have your budget set aside or perhaps you are wanting to know how much you need to set aside. There is lots to consider such as the size of the extension, quality of the build, location of the extension, access etc. These are all factors that will affect the total cost.

To help with your initial planning, we have provided some costs below on recent projects undertaken at our North Wales and Cheshire Office.

Single Story

Basic specification: £1,200 to £1,510/m²

Medium specification: £1,510 to £1,820/m²

High end specification: £1,820 to £2,100/m²

When calculating the second floor you do not double the cost of above as you are only adding more walls and floor joists, as foundations and a roof are required whether it is a single- or two-story house extension. Just add 38% to the above figures as a guide if you were to build a two-story extension.

Other costs to allow for

  1. Ground conditions – Generally with house extensions it is rare to find ground conditions dissimilar to what your existing house is built on. However occasionally you may find filled ground (full of loose material from previous use) or poor sub soil. Some on site investigation can help reduce any unknown costs that might present themselves during the build.
  2. Size – As the size of the extension increases so does the cost per square meter reduce. This is because site setup and overheads are the same for a £20,000 project as they are for a £350,000 project. Also, there will be a better rate on materials due to the volume. The contractor also has more scope to be provide a competitive cost if it is a longer project.
  3. Location – Access to site can add a significant cost to the project. Possible traffic to the site will be, skips, material deliveries from merchants, concrete mixer wagons and these days quite often a crane for lifting steel sections in. If any of the above cannot gain access, additional labour costs will be added to project to manage the process by hand or using smaller vehicles.
  4. Glazing – We have noticed this is becoming a big cost on a lot of our projects. Openings have become much larger and window sections much narrower. This has led to an explosion in aluminum window and door manufacturers offering some fantastic products but quite often equating to 12-18% of the build cost rather than 5-8%
  5. Contingency – We always recommend allowing 5-10% when working with existing buildings as you never know what you are going to find when peeling back exiting fabric. There may be existing elements that need replacing. You may also discover a sink, tap, floor tile, piece of furniture etc that you just can’t live without and that budget needs to be stretched somewhat to accommodate.

Finally, you will need to add VAT to the cost of your extension

Most projects will require vat to be paid on all invoices, which is currently 20% (April 2020). Some small contractors are not VAT registered but its very unusual these days.

Some projects are eligible for VAT relief, in the past we have spoken to the councils empty housing officer and they have provided a letter or evidence for us. The reduced rate to 5% covers renovations and alterations for residential properties that have been entirely empty for the two years immediately before work starts.