Programme
How long does an extension actually take?
The build is rarely the long part. Permissions, party wall notices and discharging conditions are what set your real start date.
The two clocks
There are two timelines on any extension, and people usually only count one. The build programme is the visible one. The approvals programme runs before it and is frequently longer.
A householder planning application has a statutory determination period of eight weeks. A prior approval application under the Larger Home Extension scheme is shorter but still real. A party wall notice under section 2 requires two months. These can run in parallel, but they cannot be skipped.
What happens after permission
Planning permissions often carry conditions requiring details to be submitted and approved before work starts — materials, drainage, construction management. Discharging those conditions is a separate application with its own timescale, and starting work without discharging them puts the permission at risk.
Building control applications should go in during this period, along with the structural engineer's calculations, so nothing is waiting on paperwork once the ground is broken.
The build itself
For a typical single-storey rear extension, the sequence is groundworks and foundations, then structure and roof to weathertight, then first fix services, then plastering, then second fix and finishes. Weather affects the first stage more than any other.
The realistic range for a single-storey rear extension is around eight to sixteen weeks on site, depending on size, ground conditions and how much internal remodelling goes with it. Anyone offering a firm figure before surveying the property is guessing.
What actually causes delay
In our experience the common causes are: ground conditions worse than expected, which is why foundations are inspected; client decisions on finishes arriving late, which stalls second fix; and materials with long lead times — structural glazing and bespoke joinery in particular — ordered too late.
The one cause we control completely is resourcing. A contractor who takes on more jobs than they can staff will leave one of them standing open, and it will be whichever one has already paid the most up front.
This guide describes national rules for England and was written to be accurate at the time of publication. Local designations can restrict them further, and legislation changes. Always confirm the position for your own address with your local planning authority, or ask us to do it in writing.
Related service: Extensions in London
Answers
Questions we get asked alongside this
Do I need planning permission for extensions in London?
It depends on the property type and what is proposed. Many single-storey rear extensions and loft conversions fall under permitted development, but those rights do not apply to flats and are restricted in conservation areas or where an Article 4 direction applies. We check the planning position for your specific address before quoting, and tell you in writing which route applies.
How do you price the work?
From a site survey, written up as a fixed and itemised scope before any deposit is discussed. If something genuinely unforeseen appears once work is opened up, it is priced and agreed in writing before we continue — never added to the final invoice after the fact.
When do I pay?
In arrears of work completed. You pay for work that has been done and inspected, not for work that has been promised. This has been how the company operates since 2008.
Do you handle Party Wall notices?
Yes, where the work requires them. Under the Party Wall etc. Act 1996, notifiable work on a shared wall needs 2 months written notice to the adjoining owner, while excavation near a neighbouring structure and building at the line of junction need 1 month. We serve the notices and manage the award process as part of the job.
Will I get certificates at the end?
Yes. Electrical work is tested and certificated under Part P, and building control issues a completion certificate where an application was required. You receive the handover pack — it is what a buyer's solicitor will ask for when you eventually sell.
How long will it take?
It depends entirely on scope, and we give a programme with the written quote rather than a guess on the phone. What we will not do is start a job we cannot resource, which is the usual reason a build stalls halfway through.
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