Start six to nine months before you want work to begin on site, and closer to twelve months if you need to be finished by a particular date. That sounds excessive until you add up the statutory waiting periods, which run one after another rather than alongside each other. A householder planning application takes eight weeks to determine once validated. Building regulations full plans approval takes up to five weeks, or two months if extended by agreement. A party structure notice under the Party Wall etc. Act gives your neighbour two months to respond. Design and drawings come before all of that, and a contractor worth having is usually booked several months ahead.
The most useful thing you can do early is establish which consent route applies, because that single decision drives every other date in the programme. Permitted development, prior approval and a full householder application have completely different timescales, and the answer depends on your property and where it sits rather than on what you want to build. From working with clients across sixteen towns in Kent, we’ve found that the projects which run smoothly are almost always the ones where that question was settled in month one rather than month four.
The timeline stacked up honestly
Here is what the calendar actually looks like for a typical house extension that needs planning permission.
Design and drawings take anywhere from four to twelve weeks depending on how decisive you are and how complex the property is. Validation of the planning application is not a statutory period and varies by authority, so allow one to three weeks before the clock even starts. Determination is then eight weeks for a householder application, with a twenty one day neighbour consultation sitting inside that window. Structural calculations and building regulations drawings usually run in parallel with the planning decision if you are organised, and the full plans approval itself takes up to five weeks. Party wall notices need serving with two months in hand for a party structure notice, or one month for a line of junction or excavation notice.
Add those up and you are at roughly five to six months before anyone lifts a shovel, assuming nothing goes wrong. Then there is your builder’s availability on top.
If planning is refused and you resubmit, add another three months and a second application fee. From 1 April 2026 a householder planning application in England costs £548, and fees now rise every April in line with CPI. There is no free resubmission, so a refusal is an expensive way to learn what the planning officer wanted.
Work out your consent route before you fall in love with a design
This is where early planning pays for itself, and it is the step most homeowners skip.
A single storey rear extension may fall under permitted development at three metres for a terrace or semi and four metres for a detached house. Going deeper than that, up to six metres and eight metres respectively, means the prior approval route under the neighbour consultation scheme, which has a forty two day determination period and its own consultation of adjoining owners. Anything two storey, or forward of the principal elevation, is a planning application.
Then there are the local qualifiers, and in our part of Kent they bite hard. On Article 2(3) designated land, which includes conservation areas and National Landscapes, the larger rear extension route is not available at all and every rear extension of more than one storey needs permission. Much of our patch sits inside the Kent Downs National Landscape, including Meopham and Wrotham, so the assumption that a six metre rear extension is available by default is wrong for a great many properties around here.
Permitted development rights can also have been removed by condition on an earlier permission, which is common on estates built in recent decades. Checking the original consent for the property takes an afternoon. Discovering the restriction after your architect has drawn the scheme costs considerably more.
We’ve found this is the most valuable half day in the whole project, and it happens best at the very start.
Party wall notices are the deadline people miss
Almost every extension on a terrace or semi touches a party wall, and the notice periods are longer than people expect.
Under the Party Wall etc. Act 1996, a party structure notice requires two months’ notice and a line of junction or excavation notice requires one month. Those cannot be compressed unless your neighbour agrees in writing. Serve them late and you either delay the start or begin without cover, which leaves you exposed if a crack appears next door six weeks in.
Serving early costs nothing and gives you a documented record of the neighbouring property’s condition before work starts. In our experience the neighbours who object are usually the ones who first heard about the project when the scaffolding arrived.
Full plans or building notice, and why it matters to your programme
You have two routes through building control, and the choice affects both your timeline and your risk.
In our experience, a full plans application works better than a building notice on an extension because the problems get found on paper rather than in the ground. With full plans you get a written approval notice before work starts, your builder prices against an approved scheme instead of an assumption, and variations shrink accordingly. With a building notice you can be on site within about forty eight hours of acceptance, which is genuinely faster, but nothing has been checked, so any non-compliance is discovered by an inspector standing in your half built extension. Correcting a foundation depth or an insulation build up at that point is the sort of cost nobody budgets for. Full plans approval also lasts three years, and lenders and conveyancing solicitors are more comfortable with it when you eventually sell.
The five week approval period is the reason this belongs in your early planning rather than the week before you start. More detail on what building control will be looking for sits in our guide to understanding building regulations for extensions.
What planning early actually buys you
Beyond compliance, there are three practical gains.
The first is contractor choice. Builders with a track record are booked out, and the homeowner who calls in January for an April start gets a much better field than the one calling in March. Warren manages and sequences every project personally rather than running several at once, so the diary is genuinely finite.
The second is a firmer price. A contractor pricing from approved drawings and calculations can give you a real figure. A contractor pricing from a sketch is guessing, and the gap between the guess and the invoice is where most extension budgets fail. If cost control matters to you, that is also the moment to read up on planning a home extension on a budget, because the savings are found at design stage, not on site.
The third is season and lead times. Groundworks in a Kent clay garden in February are slower and messier than in May. Structural steel, roof lanterns, bespoke glazing and specified brick all carry lead times that need ordering weeks ahead, and matching a brick to an existing elevation frequently takes longer than anyone allows for.
One date worth knowing about
Extensions built now follow Approved Document L, Volume 1, 2021 edition incorporating the 2023 amendments, with limiting U-values of 0.18 for new walls and floors, 0.15 for roofs and 1.4 for windows and doors.
The 2026 edition of Approved Document L, published in March 2026 alongside the Future Homes Standard, takes effect on 24 March 2027, with a twelve month transitional period running to 24 March 2028. The Future Homes Standard is aimed squarely at new homes rather than existing housing stock, but if your build is likely to straddle that date it is worth confirming with building control which edition your application falls under before you specify anything.
What we would do in month one
Establish the consent route and confirm whether permitted development rights still exist on the property. Set a realistic budget with a contingency of at least ten per cent. Get a builder in to walk the site before the design is fixed, because buildability problems are cheapest to solve on paper. Identify the party wall situation. Only then commission the full drawings.
Over thirty years in the industry, the pattern has been consistent. Extensions do not usually go wrong on site. They go wrong in the eight weeks nobody allowed for.
You can see completed projects across Kent in our portfolio, and the earlier stages of the process sit within our planning service. If you are thinking about extending and want a realistic programme before you commit to anything, get in touch for a quote and we will walk the property with you first.