How long does it really take to build an extension?

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So you’ve decided to have work done on your house. This isn’t something you do every day so it’s normal to have zero knowledge about how long these things generally take. I’ve seen hundreds of construction projects from an early idea through to the shiny polished end result, so let me give you a quick run through. 

There are several drivers in your timeline but these two are the significant players:

  1. Planning permission

This takes at least eight weeks from when your application is validated. Validation is different to the date of submission – it’s the process planning officers check they have all the correct documents they need to consider the plans. Your architect can help with this.

I recommend allowing about 12 weeks from submission to a decision.

  1. Builders

Finding a builder

There are two components to this:

  • Finding a builder

The best builders are generally busy, and lead in times could easily be three or four months. It takes a lot of work to produce a detailed quote, so – understandably – they don’t like to do this until you have planning and building regulations signed off and there is some certainty. Check out our article on finding a builder.

  • Build time

Find a builder

Obviously, this depends on the size of the job. But for most rear extensions and internal remodels including structural changes, new boilers and the like, I recommend allowing 14 to 20 weeks.

Some builders will claim it will be done in eight weeks. My experience of that is they mean the walls will be up and the roof on. You’ll be weather tight – but often, that’s only halfway there! 


Here are my suggested stages and dates below. It could be quicker – or slower – but this is a general guide:

Weeks 1 to 3 – preparation and briefing
Appoint an architect and agree on the design brief. Get a measured survey done and check the locations of sewers, trees and other constraints.

Weeks 4 to 8 – concept design
Work with your architect on several design options then review, feedback and refine until you agree on a final design.

Weeks 9 to 21 – consents
Using your agreed design, prepare and submit your consent documents – planning permission and building regulations – and manage these until decisions are made.

Weeks 22 to 26 – detailed design
Your architect will add more details to the design, which you can use for a tender document/to obtain builders’ quotes.

Weeks 27 to 31 – tender/builders’ quotes
Discuss your requirements with several builders and obtain those detailed quotes. Why not read this article from the builder’s perspective.

Weeks 32 to 36 – appoint a builder
Sign a contract and get a start date booked in. Your start date is likely to be several weeks later – the mobilisation period. In this timeline we have allowed six weeks.

Weeks 42 to 58 – the build
A good builder will keep you informed about progress against project timescales.

If you started this process – week one – in early November, you’ll be done and dusted before Christmas the following year.

That’s it – you’ve transformed your house into your dream home. You will love coming home!
dream extension

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