Extending: who to ask, in what order, before you spend anything

I extended a detached house in the south east of England. Three metres deep, nine metres across the back, with two internal walls taken out to make one open-plan kitchen. It went through as permitted development, so there was no planning application. The council side took about three weeks. The build took about six months from the day it started.

I applied for a Lawful Development Certificate anyway, and I did it for one reason: I expected to sell the house one day.

That combination is most of what matters about extending. The rules are not the hard part. Knowing which question belongs to which person, and asking them in the right order, is.

The numbers, since you will want them first

For a single-storey rear extension in England, the standard permitted development limit is four metres beyond the rear wall of the original house if the house is detached, and three metres for anything else. There is a second route, prior approval with a neighbour consultation, which takes those to eight metres and six metres, and it is not available on designated land. Height is capped at four metres. And no more than half the land around the original house may be covered by extensions or other buildings.

I built three metres, so I was inside the limit with room to spare.

Almost nobody blunders into this

There is a genre of article suggesting homeowners routinely build extensions in ignorance and get caught. That has not been my experience, and when you think about the process it makes sense.

You are spending a large sum of money. You will be speaking to an architect, who will tell you what is permitted development and what is not. You will need Building Regulations approval whatever happens with planning. And one of the first steps in any extension is contacting your local council to find out whether there is an issue with what you have in mind.

Follow that sequence and the problem mostly does not arise. Council first, then architect, then builder.

Enforcement does happen, so the risk is not imaginary. Put up a very large structure, never tell the council, never get planning, and have it fall outside permitted development, and you can expect to hear about it.

Every part of the job has its own professional

This is the part to underline.

The council is the authority on the rules and on planning. The architect is the person for design and for what is possible on your particular house. A structural engineer handles the steel and the calculations. Each of those people is genuinely expert in their bit, and none of them is expert in the others.

Which brings the question to builders. A good builder will often know what is straightforward and what is not, and a good one will say when something needs checking. But knowing planning law is not their profession. Their job is to build. Not all builders know each council's rules or the history of your particular property, and there is no reason they should.

The responsibility for making sure everything is in place sits with the owner. That sounds unfair when you are the one paying everybody else to be the expert. It is still true, and it is worth accepting early, because it is the owner who has to produce the paperwork years later.

What actually stops an extension

The thing that catches people is rarely the size of what they want to build. It is what has already been built.

I know a house that looked ideal for a side extension. Good space, an obvious spot, nothing about it looked difficult. It could not be done, because the rear extension had already used up the allowance. The permitted development calculation includes what is there, and once the land coverage limit is reached, the space in your garden is irrelevant.

A conservatory counts towards the allowance too, which catches people who have always thought of it as something separate from the house. In practice it often resolves itself, because the usual plan is to take the conservatory down and put the extension where it stood. Once it is gone, it is no longer there to count against you.

Estate agents generally do not know this, and there is no reason they would. Working it out takes measurements and the rules for that particular council. It also only becomes a live question in a narrow set of cases: a small house, a small garden, and an owner who actually wants more floor area. Most people never get near the limit, so most people never hear about it.

The four things I see go wrong are the same four every time. Failing to establish what the original house actually was. Ignoring previous extensions. Forgetting the rule about how much of the land can be covered. And overlooking local restrictions.

Conservation areas, and who gets caught by them

Local restrictions have to be checked with the council, property by property. There is no general answer, and anyone offering you one has not looked.

What surprises people is who gets caught. Not new buyers, in my experience. When you buy, the solicitor tells you, and it appears in the reports and the Land Registry details, so an incoming owner usually knows.

The person who gets caught is the one who has lived there for fifteen years, cannot remember what they were told when they bought, and suddenly decides on a big renovation. The information was handed to them once, a long time ago, when it did not matter yet.

Planning permission and Building Regulations are two different questions

This is the one people miss, and it is worth stating plainly.

Planning permission answers whether the development is acceptable in planning terms. Building Regulations deal with how the thing is built and whether it meets the technical standards. You can be completely right about planning and still have a problem, because they are separate approvals answering separate questions.

Most extensions need Building Regulations approval even when they need no planning permission at all. Permitted development is not a way around that and was never meant to be.

What it looks like when you sell

Everything above becomes real at the point of sale, which is when your paperwork gets read by somebody who is paid to be sceptical.

A buyer's solicitor will want the Building Regulations checks, and those are usually visible on the local council's planning portal, where you can see whether the work was submitted and whether it was lawful. If there was electrical or plumbing work that needed notifying, they will want that too. Guarantees and warranties where they apply. And the solicitor will normally push hard for a proper RICS survey, so that somebody independent has looked at what was built.

Missing paperwork does not automatically collapse a sale. What it does is delay it while everyone works out a solution, at exactly the point when delay is most expensive to everybody involved.

Indemnity insurance is one of the things that gets looked at, and conveyancers use it constantly. The circumstances are sometimes completely reasonable. I know of a house with a loft conversion where the roof sprang a leak and an old Velux window had to be replaced urgently. It was an emergency, so the proper installation certificate was never obtained, and indemnity insurance covered the gap at sale. Nobody had done anything wrong. The paperwork simply did not exist, because water was coming in.

When I buy a house that has already been extended, this is exactly what I go through. Every document that exists for the work. The council's planning portal, to see whether it was lawful, whether it went through full planning, and whether there were objections or issues. Then a survey, so somebody else has approved the work as well.

The certificate that costs about £300

A Lawful Development Certificate is not planning permission. It is the council confirming that what you have built, or intend to build, is lawful. It costs somewhere around three hundred pounds, and how long it takes depends on the council and on how complicated the application is, so there is no reliable number of days.

Here is the arithmetic that makes it easy. If you are spending eighty thousand pounds or more on an extension, a few hundred pounds to establish its planning status is not a lot of money, particularly if you expect to sell at some point. It buys certainty, and it hands the buyer's solicitor something concrete rather than your word.

That is why I got one for my own extension, and I did not have any doubt that the work qualified.

The rule that changed, which a lot of the industry has not caught up with

A surprising number of people in the trade still carry the old "four years and you are safe" idea in their heads. That is dangerous advice now.

For newer unauthorised building work in England, the normal enforcement period is ten years. There are transitional provisions, so work that was substantially completed before 25 April 2024 can still fall under the old four-year period. The question is never simply how long an extension has been standing. It is when the work was finished, and which of the two regimes that puts it in.

If you are buying a house with unexplained work on it, that matters. If you are the one building, it matters more, and it points at the same conclusion as everything else here. When you are spending serious money on something you intend to live in for years, do it properly. Ask the council, use an architect, get the Building Regulations sign-off, and keep the paperwork somewhere you will find it again.

When to stop trying to fit inside permitted development

One last thing, because people spend a lot of energy trying to squeeze a design under a limit.

If what you want is a straightforward single-storey extension inside the limits, permitted development is the sensible route. If it is anything else, a double storey, something large, or a side and rear extension done together and connected, then a planning application is the right road and you should take it rather than distorting the design to avoid it.

A council pre-application service is worth using sometimes. A complex or difficult site, a listed building, a conservation area, or an unusual extension are the cases where paying for the council's view before you commit to drawings makes sense.