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How long does a house extension take in London?
David Campbell
6 min read
Summary
A London extension typically takes six to twelve months from first conversation to completion. Construction is only twelve to sixteen weeks of that for single-storey work. The rest is design, consent, party wall notices and tendering, most of which runs before anyone arrives on site.
Key takeaways
Six to twelve months end to end is realistic for a London extension, with construction accounting for roughly a third of it.
Design and preparation take six to ten weeks before an application can be submitted.
A householder planning application runs eight weeks statutory, and London boroughs frequently take longer.
Party wall notices run one month for excavation near a neighbouring structure and two months for work to a shared wall, and are the most commonly forgotten item in the programme.
Tendering and contractor lead-in add four to eight weeks between consent and the first day on site.
The short answer
Six to twelve months from the first conversation to the last snag being closed, for a typical London single-storey rear extension. Construction accounts for around twelve to sixteen weeks of that, so roughly a third.
The gap between those two numbers is where most of the frustration in this process lives. Homeowners plan around the build period, because that is the part they can picture, and are then surprised by the months that precede it.
Where the time actually goes
Design and preparation take six to ten weeks. That covers a measured survey of the existing house, developing the scheme to a point where it can be tested against policy, and preparing the drawings for submission. Compressing this stage is a false economy: almost every expensive problem later can be traced to a decision taken too quickly here.
Consent takes eight weeks as a statutory minimum for a householder planning application, and London boroughs frequently exceed it. Prior approval under the Larger Home Extension route carries its own neighbour consultation period. Permitted development schemes skip this stage, which is the strongest argument for establishing your route early.
Technical design and building control take four to six weeks, running the structural design, the construction details and the building regulations submission.
Tender and contractor lead-in take four to eight weeks. Good contractors in London are booked ahead, and the ones available at a fortnight's notice are usually available for a reason.
Construction is twelve to sixteen weeks for single-storey work, and twenty to twenty-six for two-storey.
The two-month item everyone forgets
Party wall notices carry a statutory period of up to two months before work can begin, and they cannot be shortened by goodwill. On a terrace or semi they are effectively unavoidable.
The mistake is treating them as a pre-construction task. Served late, they hold up a start date that everything else has been arranged around. Served in parallel with the planning application, they cost nothing in programme at all.
The same logic applies to a build over agreement where the extension sits over a public sewer, which is common on London plots and runs on the water authority's timescale rather than yours.
What genuinely causes delay
Late decisions. Kitchens, glazing and finishes have lead times measured in weeks, and a decision deferred by a fortnight can move a completion date by a month if it lands on the critical path.
Variations. Changing the design once work has started is the most expensive way to change your mind, and it costs programme as well as money.
Unknowns in the existing structure. On Victorian and Edwardian houses, foundations, drainage and existing structure are unknown until opened up. This is what contingency is for, in time as well as budget.
Consent risk. A refusal or a difficult consultation can add months. Establishing the route honestly at the outset is worth more than optimism about it.
What a realistic programme looks like
Design and preparation, six to ten weeks. Consent, eight to twelve. Technical design and building control, four to six, overlapping consent where the route is certain. Tender and lead-in, four to eight. Construction, twelve to sixteen. Completion and snagging, two.
Add those honestly and you are at eight to eleven months for a scheme requiring planning permission, and closer to six for a permitted development scheme where the technical work can start immediately.
How to shorten it properly
Establish the consent route before designing, because a scheme drawn to permitted development limits that turns out to need an application has wasted the time twice.
Run party wall notices and any build over agreement in parallel with the application rather than after it.
Make finish decisions early, before they reach the critical path.
And appoint a contractor with a realistic start date rather than the earliest one offered. A build that begins two weeks later and runs to programme finishes sooner than one that starts immediately and stalls.
Frequently asked questions
How long does a single-storey extension take to build?
Construction alone typically takes twelve to sixteen weeks for a single-storey rear extension, and twenty to twenty-six weeks for two-storey work. That is the period on site only, and excludes design, consent, tendering and the contractor's lead-in time before starting.
How long does planning permission take in London?
A householder planning application has a statutory determination period of eight weeks, though London boroughs frequently take longer where consultation responses or officer workload delay a decision. Permitted development schemes avoid this stage entirely, which is why establishing your route early matters.
When should party wall notices be served?
In parallel with the planning application, not afterwards. Excavation near a neighbouring structure needs one month's notice, and cutting into a shared wall needs two. Neither can be shortened without written agreement, so a late notice delays a start date everything else has been arranged around.
What causes most extension delays?
Late decisions on kitchens, glazing and finishes, which carry lead times of several weeks. Then design changes after work has started, unknown conditions in an older structure, and consent difficulties. Most are avoidable by deciding earlier rather than by working faster.
Can I speed up the process?
Yes, mostly by sequencing rather than rushing. Establish the consent route before designing, run party wall notices alongside the application, make finish selections early, and appoint a contractor with a realistic start date rather than the earliest one offered.
How long before I can start if my extension is permitted development?
Around six months from first conversation, because the eight to twelve week consent stage disappears and technical design can begin immediately. Party wall notices and contractor lead-in still apply, so it removes a stage from the programme rather than removing the wait entirely.
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