How Long Does A House Extension Take In London?

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A London house extension typically takes six to twelve months from first enquiry to handover, and the build itself accounts for roughly a third of that.

The rest is planning, conditions, party wall notices, building control and waiting for a decent builder to be free.

That headline range is the same one you will find on every other page about extension timelines. What follows is the part those pages leave out, which is where the months actually go, and why the official statistics make London planning look almost twice as fast as it is.

Why does "eight weeks for planning" not mean eight weeks?

The statutory period for a householder planning application is eight weeks from the day after a valid application is received. That comes from article 34 of the Town and Country Planning (Development Management Procedure) (England) Order 2015.

Councils report their performance against that deadline, and the reported numbers look excellent. In the quarter ending March 2026, MHCLG recorded that 93 per cent of householder applications in England were decided "within eight weeks or the agreed time".

Read that phrase again. Or the agreed time.

An Extension of Time agreement is a written agreement between you and the council to stop treating the eight-week deadline as the deadline. Once it is signed, the council's performance statistics count the application as on time however long it takes. It is a perfectly legitimate mechanism. It is also doing an enormous amount of work in the published figures.

Here is what that looks like when you strip it out. Using MHCLG's Table P153, covering the twelve months to March 2026, London authorities made 44,569 decisions on non-major applications. Of those, 26,151 were decided inside the statutory eight weeks.

That is 58.7 per cent. The official London performance figure for the same period is 92.5 per cent.

So the gap between the number councils report and the number you experience as a homeowner is almost 34 percentage points. Just over a third of London applications, 35.9 per cent, ran on an agreed extension.

Which London boroughs actually decide inside eight weeks?

The spread between boroughs is far wider than anyone tells you. Calculated the same way, from the same government table, the real eight-week rate ranges from 86.8 per cent down to 11.8 per cent.

These are the twelve months to March 2026, provisional, for non-major decisions:

Islington is the clearest illustration of the problem. It reports 96.9 per cent performance, one of the best figures in London, on a real eight-week rate of 42.2 per cent. Fewer than half its decisions land inside the statutory period.

Barking and Dagenham reports 100 per cent and genuinely decides 86.8 per cent inside eight weeks. The two boroughs look almost identical in the published tables and behave nothing alike.

So if you are budgeting time for a Hammersmith side return, plan for three months of planning, not two. In Bexley or Sutton, eight weeks is a reasonable expectation.

One thing worth saying about the data. Table P153 covers non-major developments, which bundles householder applications together with minor development and small changes of use. It is the closest published proxy for what a homeowner experiences, not a householder-only figure.

Contains public sector information licensed under the Open Government Licence v3.0.

What happens between planning approval and the builder starting?

This is the stage almost nobody writes about, and it regularly costs two to three months.

Most London householder permissions come with conditions attached. Some of them are pre-commencement conditions, which means the work cannot lawfully begin until the council has approved the details. Materials, drainage strategy, construction method statements, tree protection.

Discharging those conditions is a separate application with its own clock. Under MHCLG planning practice guidance, the authority has eight weeks to decide it.

Eight more weeks. After the eight weeks you have already waited.

There is a safety valve, and it is badly underused. If the council has had your discharge application for at least six weeks, you can serve a deemed discharge notice. The condition is then treated as discharged unless the authority decides it within the later of the determination period or fourteen days from your notice.

Ask your architect at the point of approval which conditions are pre-commencement and get those applications in the same week. Treating them as paperwork to deal with later is how a September start date becomes a December one.

How long does a party wall agreement take in a London terrace?

Party wall is usually described as taking one to three months. That is roughly right as an outcome, but it hides the fact that the Party Wall etc. Act 1996 is a sequence of fixed statutory clocks, and the sequence is what determines your start date.

The notice periods, section by section:

A party structure notice, for work on a shared wall, must be served at least two months before work begins, under section 3.

A line of junction notice, for building up to or astride the boundary, needs one month, under section 1.

An excavation notice, needed if you are digging within three metres of a neighbouring structure and deeper than its foundations, also needs one month, under section 6. Most London extension foundations trigger this.

Then the response window. Your neighbour has fourteen days to consent in writing. If they say nothing, section 5 treats that silence as dissent, and a dispute is deemed to have arisen automatically.

Silence is not neutral. Silence is a no.

Once a dispute exists, surveyors are appointed and an award is drawn up. If a party fails to appoint a surveyor within ten days of being asked, the other party can appoint one for them. The award can then be appealed to the county court within fourteen days of service.

Now apply that to a Victorian mid-terrace, which is the typical London side return job. You have two adjoining owners, not one. Each gets their own notice and runs their own fourteen-day clock, and each can dissent independently and appoint their own surveyor.

If either neighbouring property has been converted into flats, the position gets more involved again, because the freeholder and each leaseholder can qualify as an adjoining owner. Three or four separate parties on one side of the wall is not unusual in Hackney or Lambeth.

The practical rule is simple. Serve notices two months before you want to break ground, not two weeks after planning lands.

How long does building regulations approval take?

Timings for this stage are quoted wrongly almost everywhere, with ranges from two weeks to eight weeks in circulation.

The actual position, per the Planning Portal, is that with a full plans application the authority must issue a decision within five weeks, or a maximum of two months if you agree to the extension.

The route you choose matters more than the wait. A full plans application gets your drawings checked before anyone is on site, which means your builders are pricing something that has already been approved. A building notice skips the pre-approval wait, but it moves the risk onto the site, where changes cost real money.

For an extension being competitively tendered, full plans is almost always the better trade. You cannot get like-for-like quotes on drawings that might still change.

Two administrative details that catch people out. You must give a commencement notice, and work cannot start until at least two days have elapsed since the end of the day you gave it, under regulation 16 of the Building Regulations 2010. And you must notify completion within five days of the work finishing.

How far ahead are London builders booked?

Here is the queue nobody puts in the timeline.

Tendering itself takes two to six weeks: issuing drawings, fielding questions, getting priced returns back, comparing them properly. That part is manageable.

The part that is not is the lead-in. A good London contractor with a full book is frequently quoting start dates three to six months out. On a well-run project this runs in parallel with planning and party wall, which is exactly why you should be tendering while you wait for a decision rather than after it.

The cheapest way to lose four months is to treat the stages as a strict queue.

It is also the stage where the most expensive mistakes get made, because a quote that comes back well under the others is usually pricing a different job, not a better-value one. Firms that work on a fixed price from a single agreed scope, like the London design-and-build team at Beams Renovation, give you a like-for-like comparison and a start date you can actually plan the rest of the programme around. The alternative is three quotes that cannot be compared, which is how budgets drift by twenty per cent before anyone has dug a hole.

What does the build itself take?

On-site durations vary by extension type, and these are trade norms rather than published data, so treat them as planning assumptions:

A single-storey rear extension typically runs ten to fourteen weeks. A side return on a Victorian terrace is similar, at ten to fourteen. A wraparound extends to twelve to sixteen. A double-storey extension usually lands somewhere between fourteen and twenty.

Inside that, the sequence is fairly predictable. Groundworks and foundations, then the structural shell, then getting it watertight with the roof and windows in. First fix for electrics and plumbing, plastering, second fix, then decoration and floor finishes.

The two stages that slip most often are the groundworks and the second fix. Groundworks because London clay and shallow Victorian foundations produce genuine surprises, and because the section 6 three-metre rule can trigger underpinning nobody priced for. Second fix because it depends on trades, materials and client decisions arriving in the right order.

When is the job actually finished?

Practical completion is not the end, and the difference matters if you ever want to sell or remortgage.

Snagging usually takes one to three weeks of working through the defects list after the builders are nominally done. Then there is the building regulations completion certificate, which is the document your lender and any future buyer will ask for. It is not the same thing as the builder leaving.

Retention is the other piece. A typical five per cent retention is held for a defects period that commonly runs six to twelve months, with the final account settled at the end of it.

So "handover" has at least three meanings: the day you get your kitchen back, the day the paperwork is complete, and the day the last money changes hands. They can be a year apart.

What goes wrong, and what does each failure cost in weeks?

Delays are usually described as a vague list of risks. It is more useful to know what each specific failure costs you.

An invalid application costs two to six weeks, and it is more common than people expect. The eight-week clock only starts on a valid application, so a validation loop is time that never appears in any official statistic.

A refusal is the expensive one. A householder appeal by written representations took a median of 12.6 weeks for decisions issued in March 2026, according to the Planning Inspectorate. Resubmitting instead is free within twelve months, but it restarts the eight weeks from scratch. London has the lowest grant rate of any English region, at 81 per cent in the quarter ending March 2026, so this is not a remote risk.

A deemed party wall dissent costs whatever the surveyors take, and it starts automatically on day fifteen of your neighbour's silence.

Unknown ground conditions are the wildcard, and underpinning is the usual culprit in older London stock.

What about conservation areas?

Conservation areas are always named as a delay factor and almost never explained.

The concrete change is this. The larger home extension prior approval route, which handles a deeper rear extension in 42 days under the General Permitted Development Order 2015, is not available on article 2(3) designated land at all. Conservation areas are article 2(3) land.

So a rear extension beyond three metres, or four for a detached house, goes to a full householder application in a conservation area rather than the quicker prior approval route. Many inner-London boroughs also have Article 4 directions that remove further permitted development rights street by street.

Check whether an Article 4 direction applies to your specific address before you assume anything is permitted development. They are not borough-wide.

What can run in parallel?

Almost every timeline you will read adds the stages up in a straight line. Real projects overlap them, and that overlap is the difference between eight months and fourteen.

These can run concurrently: building regulations drawings and structural design alongside the planning determination; party wall notices served while planning is pending; tendering and contractor selection during the wait.

These genuinely cannot be compressed: pre-commencement conditions must be discharged before a lawful start; party wall notice periods must expire before excavation; the two-day commencement notice must elapse.

Sequence everything else around those three fixed points.

A realistic London programme

Pulling it together for a typical single-storey rear or side return extension in a London borough with average planning speed:

Feasibility and measured survey, two to four weeks. Design and drawings, four to eight weeks. Planning determination, eight to sixteen weeks depending on the borough. Discharging conditions, four to eight weeks, overlapping with building regulations at five weeks. Party wall, two months of notice period, ideally served during planning. Tender and builder lead-in, six to twenty-four weeks, mostly overlapping. Build, ten to fourteen weeks. Snagging and certification, two to four weeks.

Run sequentially, that is well over a year. Run properly in parallel, with the fixed points respected, eight to ten months is achievable.

The variable that moves your date most is not the build. It is which borough you are in, and whether you started the party wall and conditions clocks early enough.

Sources

Town and Country Planning (Development Management Procedure) (England) Order 2015, article 34, for the eight-week statutory determination period and extension of time provisions.

MHCLG, Planning applications in England, January to March 2026, published 19 June 2026, for national householder decision volumes and London grant rates.

MHCLG live tables on planning application statistics, Table P153, twelve months to March 2026 provisional, for borough-level decision counts. Real eight-week rates calculated from the published counts.

MHCLG planning practice guidance, Use of planning conditions, for the eight-week condition discharge period and the six-week deemed discharge route.

Party Wall etc. Act 1996, sections 1, 3, 5, 6 and 10, for notice periods, the fourteen-day response window, deemed dissent, surveyor appointment and appeal periods.

Planning Portal, full plans building control applications, for the five-week and two-month decision periods.

Building Regulations 2010, regulation 16, for commencement notice and completion notification requirements.

Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 1, for larger home extension prior approval periods and article 2(3) land exclusions.

Planning Inspectorate statistical release, 23 April 2026, for householder appeal timescales.

Published: September 22, 2026