Building an extension timeline

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

Works will vary depending on the type of extension, 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.

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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. 
• The roof structure and roof covering come next.
• 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 everything getting 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 extension planning guide 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

Need a good builder?

The best place to start is to try and find someone who has been through the process before and get their recommendations for good local architects, structural engineers and builders.

Which? Trusted Traders has a great library of vetted and reliable traders in your local area.