“Should we move or should we extend?” A lot of our first meetings start with that, usually at the kitchen table, usually with a toddler somewhere underneath it.
We’re builders, so you’d expect us to say extend. Fair. But we’ve also told people not to bother, because the house or the garden or the budget just wasn’t going to give them what they wanted. So take this as the version we’d give a mate, not a sales pitch.
Why so many people extend instead of moving
Have a look at what moving actually costs in London. On a £1 million house, stamp duty alone is £43,750 if it’s your main home. Then there’s the agent, the solicitor, the survey, the removal van, and the money you’ll spend in the first year putting right whatever the last owners did. That’s a big pile of cash that buys you no extra space at all.
Put the same money into a house extension and you keep the street, the school run and the neighbour who takes your parcels in. You also get to decide how the space works, instead of living with somebody else’s idea of a kitchen. Most of the time people want one of a handful of things: a bigger kitchen extension the family can actually eat in, somewhere to work that isn’t the end of the bed, or one more bedroom.
It doesn’t always add up, mind you. If you need two more bedrooms and the house is a two-up two-down with a tiny yard, moving might honestly be the better call. We’d rather say that at the start than halfway through a build.
The main types of extension
London is mostly terraces and semis, so honestly the same four or five designs come up again and again.
The one we build most is the plain rear extension, out into the garden. You knock through the back wall, put a kitchen-diner across the full width with big sliding doors, and the ground floor finally makes sense. Out of all of them it’s the least hassle to live through. Light comes in through the doors and through roof lanterns or rooflights, which you’ll want, because the old back room is about to become the middle of the house.
Then there’s the side return. If you’ve got a Victorian or Edwardian terrace you know the bit: a strip down the side of the kitchen with the bins, a drainpipe and maybe a sad fern in a pot. It’s often barely more than a metre wide. People can’t believe how much difference it makes until the wall comes down and the galley kitchen is suddenly a room you can put a table in.
Do both together and you’ve got a wraparound, an L shape round the back corner. It costs more and there’s more steel in it, but if you asked us which single job changes a ground floor the most, without digging, that’d be the one.
Double storey is a different conversation, because you’re adding a bedroom or bathroom upstairs as well. More money, harder planning, but you only pay for one set of foundations and one roof, so per square metre it’s actually the better deal.
Basements? They do work. They’re also slow, very expensive and properly messy, and we’d nearly always look at the loft first.
Single storey or two?
Single storey is simpler. Quicker, cheaper, less to go wrong, and if what’s bugging you is the kitchen and the lack of light downstairs, it’s often all you need.
Two storeys only really make sense if you need bedrooms and you’re planning to stay a good while. You’ll pay more for it and planning is tougher, but nobody we’ve built one for has ever wished they’d gone single storey instead.
Planning, permitted development and the paperwork
Here’s the short, slightly less scary version.
A lot of single-storey rear extensions don’t need full planning at all. They go under permitted development: on a terrace or semi you can generally go 3 metres out from the original back wall (4 on a detached house), up to 4 metres high, and no more than 3 metres at the eaves if you’re within 2 metres of the boundary. There’s also a prior approval route that lets you go deeper, up to 6 metres on a terrace or semi, as long as the neighbours don’t kick up a fuss.
Where it gets harder: conservation areas, listed buildings, flats (they don’t have PD rights at all), and most double storey or wraparound designs. Those normally need planning permission, and in a conservation area the council can be quite particular about brick, windows and roof shape.
Two other things that apply to nearly everyone. If you’re building on or near a shared wall, you’ll need to deal with the Party Wall Act, which means notices to the neighbours and sometimes a surveyor each. And every extension needs Building Regulations sign-off, PD or not.
Even if you’re sure it’s permitted development, we’d still apply for a Lawful Development Certificate. It’s not compulsory, but it’s the bit of paper your buyer’s solicitor will ask for when you sell, and it’s a lot easier to get now than in ten years’ time.
What it costs
At the moment, most London extensions come out somewhere around £2,500 to £3,500 per square metre for the build, depending on the spec, the structure and how easy it is to get onto the site. On top of that come architect and engineer fees, building control, party wall surveyors if you need them, the kitchen itself, and VAT.
Then keep 10 to 15 per cent back as contingency. We know nobody likes hearing that. But these are London houses, a hundred-odd years old in a lot of cases, and when you open them up you find things: foundations that are a couple of bricks deep, a drain running exactly where the new wall needs to go, a bit of old wiring that frightens the electrician.
The things that push the price up are pretty predictable. Big steels and anything structural. No side access, so every bag of cement and every bit of rubble goes through the house. Conservation area hoops. And glazing, which catches people out more than anything, because a really good set of slim aluminium sliders and a big frameless rooflight can cost as much as the kitchen.
A detailed quote helps more than anything else. If a price says “kitchen extension, £60k” and nothing else, you’ve got nothing to hold anyone to.
Making it look like it belongs
The best extensions don’t look stuck on. That’s mostly about respecting the house: bricks that match or deliberately don’t, rooflines that line up, windows in proportion with the ones you already have. Inside, carry the same flooring through so the old and new rooms read as one space.
And plan it around how you actually live, not how the photos on Instagram look. Where does the shopping get dumped? Where do the kids do homework? Where does the dog sleep? A good layout answers those before anyone draws a single pretty sketch.
A lot of people want completely open plan, and then find it’s loud, echoey and smells of whatever was cooked last night. “Broken plan” is usually the better answer: one big space, but with a change of floor level, a bit of joinery or a half wall to give it some zones.
Keeping it warm (and the bills down)
Building regs already set a decent minimum for insulation, but it’s worth going a bit better if you can. Good glazing, an airtight build, underfloor heating that runs off a heat pump or your boiler. If you’re putting in a lot of glass, think about which way it faces, because a south-facing glass box can be roasting in July.
It all adds to the bill up front, sure. But you’ll notice it every winter you live there.
Picking a builder
Get proper itemised quotes. Ask to see their insurance, find out what guarantee they give, and ask who’s actually going to be running your site day to day, because it’s often not the person who came round to quote. Go and see a finished job if you can and talk to the owners.
Then trust your gut. You’ll be seeing these people most mornings for three or four months. If the first few conversations are hard work, the build will be too.
A local builder helps more than people think. They’ll know how the borough’s planners tend to react, where the clay is bad, which streets are a nightmare for skips, and they can be round in twenty minutes when something needs looking at.
How a build usually runs
The part nobody expects is how long it takes before anything happens on site. Survey, drawings, planning or a Lawful Development Certificate, the structural engineer, party wall notices to the neighbours, building control. Three to six months of that is normal, and it’s frustrating, because there’s nothing to look at.
Once we’re on site it’s demolition and foundations first, then drains (always the drains), walls and steels. Getting the roof on and the glazing in is the moment it starts to feel real, because the house is watertight again. After that the electrician and plumber do their first fix, it gets insulated and plastered, and then comes the bit everyone’s been waiting for: the kitchen, the floors, the lights, interior decorating.
Last of all is snagging. Don’t rush it, even though by then you’ll be desperate to have your house back.
The usual mistakes
If there’s one we see more than any other, it’s the budget that forgot VAT. Twenty per cent on a £90k build isn’t a rounding error, and it’s amazing how often it’s missing from the sums people bring to the first meeting.
The other big one is changing your mind late. Moving a doorway on the drawings costs nothing. Moving it after the steels have been made and delivered costs a lot, plus a week or two while new ones are fabricated. Decide early, even if it feels like you’re rushing it.
Extension or loft?
Both can add good value. Estate agents generally reckon a loft conversion with a bedroom and bathroom adds something like 15 to 20 per cent, and a well done extension anywhere from 10 to 25 per cent, depending on the area and how much it changes the house. Those are rough numbers, not promises.
The real question is what’s missing. Not enough bedrooms? Look up. Ground floor too dark and pokey? Look out the back.
What we’ve built
We’ve done everything from glass-heavy rear extensions in Chiswick to brick wraparounds in Richmond, and the bit we enjoy most is going back a year later and seeing the space being lived in properly. Kids’ drawings on the fridge, muddy boots by the doors.
If you’re weighing up moving against extending, get in touch. We’ll come and look at the house and tell you straight what’s possible, including if we think you’d be better off moving.
FAQ: Quick Answers
Do I need planning permission? Often not for a single-storey rear extension, which can usually go under permitted development. Double storey, wraparounds, flats and anything in a conservation area normally do. Check before you get too attached to a design.
How much does it cost? Roughly £2,500 to £3,500 per square metre in London for the build, plus fees, VAT and a contingency. Structure, access and the glazing you pick make the biggest difference.
How long does it take? Around 8 to 12 weeks on site for a typical single-storey extension, and 12 to 20 weeks or more for double storey or complicated jobs. Design and approvals come before that and can take months.
Can I live at home during works? Usually, with a rear or side extension. It’ll be noisy and dusty, and there’ll be a few weeks with a temporary kitchen somewhere. Lots of families plan a holiday around the knock-through.
How do I make it efficient? Better insulation than the minimum, good glazing, an airtight build and underfloor heating with decent controls. It costs a bit more now and pays you back every winter.
Thinking about extending your home?
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Force Builders Ltd is a London building company (Companies House 07049787) delivering house extensions, loft conversions and full refurbishments across Greater London. See our projects or get in touch.