We use cookies to allow us and selected partners to improve your experience and our advertising. By continuing to browse you consent to our use of cookies. You can understand more and change your cookies preferences here.

Coronavirus Read our latest advice
Reviews based on facts
Our rigorous tests find the facts, and our impartial reviews tell you the truth about how products perform. First month £5, then £9.99 per month, cancel anytime.
Try Which?
1 October 2020

Building an extension timeline

Make sure your project runs smoothly with the help of our step-by-step extension planner.
RC
Rachel Christie

Read through our extension checklist to keep on track and make sure nothing's forgotten! 

Works will vary depending on the type of extension you're having done, but a timeline will give you a better understanding of what's involved. 

From laying down the foundations to finishing the plastering, here's a general works plan for a standard single-storey extension:

First fix

This refers to everything before the point of finished plastered walls.

Weeks 1-2

  • Builders will arrive on site.
  • Preparation begins for the groundwork and foundations.

Week 3-4

  • Superstructure starts, which includes building up a damp-proof course (DPC).
  • Drains are installed.
  • Lintels are fixed in place.
  • A concrete floor and insulation is put in place. 

Week 5

  • The building of external walls begins, which includes block work or brickwork
  • Cavity wall insulation will be fitted.
  • Wall tiles are then used to fix the new walls to those in the existing house.

Week 6 - 7

  • Internal walls are built at this stage. 
  • Followed by the roof structure and roof covering.
  • A carpenter will build the roof structure. 
  • Roofing membrane will then be laid over the rafters before roof battens are fitted in place.

Week 8

  • Windows and doors will now be installed. 

Week 9

  • First fix carpentry, plumbing and electrics begin. 
  • The extension should be watertight by now and the three trades work closely together at this stage to ensure each knows what the other is doing. 
  • Stud work might begin now.

Week 10

  • Break through from the existing house and steels are put in place.
  • This can be a very messy time, so if you're living at the property or just don't want your things to get covered in dust, it's a good idea to seal everything off as much as possible.

Week 11

  • Plastering, followed by a period of about a week for drying out.

Second fix

This refers to everything that comes after plastering has finished.

Week 12-13

  • Second fix electrics meaning sockets will be made live, switches put into place and lights will be fitted. 
  • Second fix plumbing where taps are installed and connections completed.
  • Flooring will be fitted and kitchen units installed, if applicable. 

Week 14

  • Snagging, which means any leaks, problems with electrics and anything that's not quite right should be reported to the builders as soon as possible after finishing. 

Visit our page on extension planning for a more detailed look at whether investing in an extension is right for you, what permissions and insurances you might need and what type of extension to go for.

LATEST NEWS IN Which? Home improvements See all News