How Long Does an Extension Take to Build? The Timeline

Extension build times by type of project

Each time below comes from the Checkatrade guide for that type of project.

Project	Typical time	Source
Single storey extension	3 to 6 months from groundworks to completion	Checkatrade planning guide
Double storey extension	8 to 14 weeks of construction	Checkatrade cost guide
Garage conversion	Around 2 to 4 weeks	Checkatrade cost guide
Removing a load-bearing wall	Usually 2 to 3 days	Checkatrade cost guide

Most single storey extensions take 3 to 6 months from groundworks to completion, depending on the size, the weather and how quickly materials and tradespeople can be scheduled, Checkatrade’s single storey extension planning guide says.

Most double storey extensions take 8 to 14 weeks of construction, and the designs, drawings, planning permission and structural planning before it take from 2 weeks to several months, Checkatrade’s double storey extension cost guide says.

If you are weighing up a garage conversion instead, most take around 2 to 4 weeks, Checkatrade’s garage conversion cost guide says. If your plans also take out a load-bearing wall, that part usually takes 2 to 3 days, Checkatrade’s load-bearing wall guide says, with 2 days for the steel beam, the removal and the waste, then plastering and finishing the next day, and electrical or plumbing work adding time.

Extension delays and comparing build times

Checkatrade’s house extension cost guide puts an extension at around 3 to 6 months to build, depending on its size, planning difficulties, materials and the weather, and says delays are very common, especially with several contractors.

The build times builders give you only compare if each one is quoting for the same extension. The first step in Checkatrade’s planning guide is a brief saying why you want the extension, which it says helps when you compare quotes. Send every builder the same brief, and the price of the extension and its build time come back for the same job.

The extension timeline before building starts

Before the first day on site, an extension can need prior approval, a building regulations application and party wall notice, and each has its own period.

Stage	When it applies	Period	Source
Prior approval	A larger single storey rear extension	A 42-day determination period, with at least 21 days for neighbours to object	Planning Portal, guidance note
Building regulations, full plans application	Most extensions	A decision within 5 weeks, or 2 months with your consent	Planning Portal, GOV.UK
Party wall notice	Work to an existing party wall	At least 2 months before the work	GOV.UK
Party wall notice	A new wall at the boundary, or excavating within 3 or 6 metres of a neighbour’s building, depending on the depth	At least 1 month before the work	GOV.UK

Permitted development and prior approval

Many extensions need no planning application at all, because permitted development rights allow them within set limits, the Planning Portal says, and beyond those limits an application for householder planning permission is likely to be needed.

A larger single storey rear extension, over 4 and up to 8 metres beyond the rear wall of a detached house or over 3 and up to 6 metres on any other house, can go through prior approval instead, the Planning Portal says. That route is not open on designated land such as a conservation area, or in a Site of Special Scientific Interest.

You apply before work starts, since the process cannot be done retrospectively, the Planning Portal’s guidance note says. The work can go ahead once the council confirms in writing that prior approval is not required or that the effect is acceptable, or if no decision is notified within the 42-day determination period.

Building regulations approval and the completion certificate

Most extensions need approval under the building regulations, the Planning Portal says. You can apply to your local council or a private registered building control approver, and with a full plans application, the most thorough option, you can expect a decision within 5 weeks, or 2 months with your consent, GOV.UK says.

The completion certificate is the last stage on the timeline, usually arriving within 8 weeks of the building work being completed, as long as it complies, GOV.UK says.

Party wall notice periods

A party wall notice is only valid for a year, GOV.UK’s explanatory booklet on the Party Wall etc. Act 1996 says, so time it against your planned start. If your neighbour has done nothing 14 days after it is served, a dispute is deemed to have arisen, and one agreed surveyor, or one surveyor each, draws up an award.

You wait out the notice period unless your neighbour agrees in writing to an earlier start, and work started without proper notice can be stopped by a court injunction, the booklet says.

Agreeing a start date with your builder

Ask each builder you are comparing which of these stages it will handle, from the drawings to the completion certificate, and how many weeks it expects to be on site. Among the Nottingham builders we compare on extensions and new builds, D&M Building Services builds single and double storey extensions with an in-house construction team supported by specialist subcontractors, and works to detailed quotations with a defined scope and staged payments. Edge Building & Construction, which builds rear, side and wrap-around extensions, has a team of architects, interior designers, structural engineers and contractors that takes a project from conception to completion.

Before a start date goes into the contract, work through each stage your extension needs.