Building a house extension: foundations to finished room

This is an extension I built for my sister, filmed the whole way through. The video above is the finished thing set to music, so if you want to know what actually happened at each stage, that’s what this page is for.

It’s a good one to read before you have an extension built, because most of the work that decides whether it goes well is buried in the ground or hidden in a cavity by the time anyone sees the room.

The foundations had to be piled

The ground wasn’t good enough for a normal footing, so it got piled. Hollow steel posts driven in at certain points, a cage of steel bars over the top of them, and the lot concreted in. That ties everything together and reinforces it.

There was another problem. Underneath the middle of it there was a mass of old concrete, six or seven foot deep. You’re not taking that out by hand, so the answer was to bridge across it with the piles. The layout also shifted over by about 100 mil, because moving a drainage grid that was set in all that concrete would have been far more hassle than moving the extension.

The building inspector agreed to that. That’s the point worth taking from this: the foundation design comes from the engineer and gets signed off by Building Control on your job. It isn’t something to copy off a video.

Setting out against the house

First thing I do is strike a line through, so I’m working off the gable of the existing house. Once you’ve got that, everything else works off it.

Below ground it’s a sequence: crushed run, sand blinding, the damp-proof membrane, insulation, then the steel reinforcement mesh. Each sheet of mesh weighs 70 kilograms, and it gets propped 50 mil off the floor so the concrete can get underneath it as well as over the top. We put a duct in for the electrical cable while we were there.

The brickwork under the slab goes up in engineering brick outside with three courses of commons inside. The slab sits on those at 150, and that brings you up to damp course level.

The inspector came back again before the pour to check it. Then it was shuttered in OSB, packed with 100 mil insulation, and poured. Mix was three to one. You tamp it with a seesaw action, taking off and scraping, levelling as you go.

Cutting the cavity in

On the brickwork, the first job is cutting a 100 mil cavity in so the existing cavity carries on into the new extension.

The damp course goes underneath, then the weeps go in. They’re there so that any moisture that does get into the cavity has somewhere to drop out. I put them in the joints every four bricks or so: the vent goes in dry and the next brick comes up to it. On the foundation job, Building Control told us exactly how they wanted them in.

The wall starters go on with damp course behind them, and that’s a requirement, not a nicety. It’s lapped one way and not the other, so anything running down the cavity runs out rather than in.

One small thing that saves you all day: polycarbonate sheets for spot boards. They don’t suck the water out of your mortar, and in that weather it was going off fast enough already.

Brick, block, ties and gauge

We got the blockwork up eight courses and then jumped onto the brickwork. Ties went in every two courses, staggered. Staggered just means offset instead of sitting on top of each other, so you get a diamond pattern rather than a column of ties.

Keep it gauged and you’ll be level, as long as your footing is level. Gauge off something you know.

A few things that matter with block:

• Level them and plumb them. Every one.
• A nice 10 mil bed, and get your perps across before you knock the block down.
• Push it down and give it a couple of rubs. Don’t leave it sitting on the mortar.
• Keep checking your cavity at the top. Concrete block can creep it out on you if you’re not watching.
• Keep the blocks dry. A wet block slides about and you can’t push it properly.

The facing bricks on this one had a front and a back, strangely enough. Sharp edge on the front, rounded edge on the back. After a bit you don’t look, you can feel it when you pick one up. There were three different colours in the load as well, so you keep alternating them as you go up instead of ending up with a patch of one colour.

Laying in the heat, and jointed or raked out

When I filmed the first part it was hot and the mortar was drying like mad. Out of the mixer it’s already going off, so you’re going like the clappers to stay in front of it. We only got up five courses that day.

That gave me a decision to make, and I put it to the customer. If I jointed it, I’d have to stop and joint nearly every course, which would have added something like three extra days. If I raked it out instead, I could build the whole wall and rake it out at the end.

So I jointed half and raked half out and asked which they’d have. If you’re having an extension built, that’s a call you get a say in, and it’s worth asking about rather than finding out at the end.

People ask how you know when to joint. You judge it on the job, on the weather and the mix. Generally it’s when you can just see your thumbprint in it without the mortar dragging.

Where blockwork was getting rendered we didn’t point it at all. We roughed the joints and filled them so the render keys properly, and flushed the outside up. A good key stops the render coming off and stops it ghosting through.

Up to the steels

We took the brickwork up to 2.4 all the way round. Behind it there’s the padstone and a course of commons.

One bit sat a touch high, so it had to be trimmed off to get a bed under the steel. Bearings, padstones and the courses over them follow the engineer’s design on your job, and that’s a detail to get checked rather than guessed.

The lintel over the heads of the doors was solid, properly supported and straight as an arrow. That’s what you want, because the doors are big sliders, two sections of glass with one half sliding onto the other, and a big opening that’s out by a little is a nightmare to fit and adjust later.

The roof and the rooflights

The roof starts with the timber. Firrings go on to give it the fall from the front of the extension to the back, then tongue and groove decking on top of that, joints taped with foil tape and the lot screwed down.

The rooflight frames go in before the glass, and you check your diagonals on them. The glass has to sit square, and there’s no adjusting it once it’s up there. At both ends that glass is carried by the steel, which is why the steel is designed the way it is on that side.

Then the fibreglass. Chopped strand mat, resin and catalyst, topcoat over it, and an angle trim round the perimeter first. It was 30 to 34 degrees in Wigan that week, so it went off nicely, and a second coat of topcoat went on afterwards to make it smooth.

It took four lads to lift the rooflight glass in. Before it goes on you trim the resin residue off the frame, or the glass can crack on it, then seal it with a proper silicone.

What this job tells you about quotes

An extension is a chain. Bad ground changes the foundation. The foundation decides where the walls land. The walls set the openings and the roof levels. The roof and the steel decide how the glass goes in. Rush any one of them and it turns up later, when it costs more.

It’s also why two extension quotes can look nothing alike. A piled foundation isn’t priced like a strip footing, and a roof with rooflights and big sliders isn’t priced like a plain flat roof. One of those quotes might not be expensive. It might just be the only one that’s read the ground.

Watch the job

• Building a Luxury House Extension From Start to Finish, the finished build, set to music
• How To Do Foundations For House Extension, the piles, the prep and the pour
• Building House Extension, Series UK #1, cutting the cavity in and starting the brick and block
• Building an Extension, part 3, up to 2.4 and the steels going on
• GRP Roof Done and Rooflight Glass Installed, the roof, the fibreglass and the glass going in

Having an extension built?

Before you agree to anything, ask what foundation has been allowed for and who designed it, how the cavity ties into the existing house, what finish the joints are getting, how the steel is supported, and exactly what’s in the price for the roof and the glazing.

If a quote can answer all five, it’s a real quote. If it can’t, it’s a number.

Post your extension free on Hard Hat Way and trades near you can see the job and quote. The extension materials calculator helps with early quantities, and the single-storey extension brickwork cost guide covers the wallwork end of it.