Week by Week Guide to Building an Extension

This page is part of our new blog and is all about house extensions.

With the housing market being so volatile and sky-high mortgage fees and stamp duties, more and more homeowners are looking to extend rather than relocate.

The government has even relaxed some of the planning regulations for straight-forward extensions.

But how long does it take to build a typical extension of around 25 square metres?

The guide below is for a side or rear extension. We’ve created different guides for loft conversions, porches and conservatories.

How Long Do You Think a 25 Square Metre Extension Will Take to Build?

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The question below is optional, you can skip it if you wish:

Week 1 – Preparation

  • Inform neighbours that the work will be starting soon and ask for their patience during the project. Upsetting the neighbours can often lead to complaints and denial of access which could delay the work and add extra stress.
  • Make sure access to the site is clear and fence off any areas that children or pets will be using during the project.
  • Create a space for the materials to be stored; this should ideally be close to the work area and secure.
  • Create a space for the skip.
  • Arrange delivery of the first batch of materials.

Week 2 – Foundations and Groundwork

  • The builders will arrive to start excavating the ground for the foundations.
  • Reinforcement bars will then be inserted into the foundations if they’re needed.
  • Arrange for Building Control to check the foundation depth, width and reinforcements.
  • Services, piping, wiring and drainage to be installed.
  • Concrete will then be poured into the foundations.
  • Arrange for Building Control to approve the foundations.

Week 3-4 – Ground and Low-Level Work

  • Trenches and drains will then be dug.
  • Bricklayers will then begin constructing the wall up to the damp course.
  • The damp course, lintels and insulation are then installed.
  • Concrete for the floor is then poured and allowed to set.

Week 5-6 – External and Internal Walls

  • Bricks are laid.
  • Wall ties used as needed.
  • Cavity wall insulation inserted.
  • Lintels installed.
  • Windows frames fitted.
  • Door frames fitted.
  • Internal blocks laid.

Week 7 – Build the Roof Structure

  • Build the roof frame.
  • Install roof windows or domes as required.

Week 8 – Lay the Roof Covering

Week 9-10 – Guttering, Windows, Doors and Render

  • Windows and doors to be fitted to the frames.
  • External walls rendered, if applicable.
  • Guttering and rainwater pipes fitted.
  • First fix carpentry.
  • Electric wiring and plumbing installed.

Week 11-12 – Knock Through and Plastering

  • Knock through into the existing building to create an opening.
  • Lintels and padstones installed.
  • Arrange for Building Control to inspect the lintel/padstones.
  • Boxing in as required.
  • Plasterboard and plaster.

Week 13 – Final Work

Week 14 – Snagging and Garden Work

  • Any minor issues to be fixed by the relevant trades.
  • The garden to be finished off neatly.
  • Final tidy up.

Delays and Hold Ups

Delays and hold-ups usually occur as a result of:

  • Trade delays – with up to half a dozen different trades working on the project, delays are inevitable. Some tradespeople won’t be able to start work until specific stages of the project are completed. Thus a delay early in the work can have a significant knock-on effect.
  • Not having an architect (see typical architect and engineer’s fees here) can lead to simple errors that result in delays further down the road.
  • The unexpected can happen, and we always suggest you keep some money to one side as a contingency.

How Much Does it Cost to Build an Extension?

If you like to know how much it currently costs to build an extension on a per square metre basis, then head over to this page.


When you ask builders how long it takes to build an extension, you’ll probably get the best-case scenarios. After all, it’s in their best interests to paint a rosy picture.

In our experience, it typically takes just over three months for a small straightforward extension and around four months for a slightly larger one.

It’s almost impossible to predict how long a complex development will take, but if your builder gives you a timescale, add an extra 20% to it, and you probably won’t be far off.

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