House extensions & structural alterations in Bedfordshire, Hertfordshire & Buckinghamshire
Almost nobody actually wants an extension. They want a kitchen big enough to eat in, a room that isn’t dark by mid-afternoon, or somewhere to put a growing family without moving house. The extension is only the route there — and how big a job it turns out to be has more to do with what is under the ground and inside your existing walls than with the footprint you drew on the back of an envelope.
The shapes this job takes
Most of what we are asked for falls into one of these. They are listed roughly in order of how much structure is involved, which is not the same as how much difference they make to the house.
- Single-storey rear. The back of the house pushed out into the garden, usually to make one big kitchen and dining space out of two cramped rooms.
- Side return. The narrow strip down the side of a terrace or semi, taken in and roofed over. Small on a drawing, and the change to the room it joins is out of all proportion to the floor area.
- Wrap-around. Rear and side together. More roof, more structure, and much more of the existing back wall coming out.
- Two-storey. A bedroom as well as ground-floor space. Deeper foundations, a bigger roof to tie into the existing one, and permission is less likely to be automatic.
- Alterations without extending. Sometimes the space is already there and simply in the wrong shape — a wall out and a steel in, a garage or outbuilding converted, a whole floor reconfigured. It is a smaller job than building, and worth ruling in or out before you commit to anything.
What makes one extension a bigger job than another
Two extensions with the same floor area can be very different jobs. This is where the difference usually lives.
- The ground. Foundations are dug to what the ground and the trees nearby demand, not to a standard depth. Clay, made-up ground, a mature tree next door or a drain running exactly where the footings want to go all change the digging before a single brick is laid.
- Drains and services. A manhole or a shared sewer under the new footprint is common and entirely solvable, but it is work — and it is far better found during the survey than on the morning the digger arrives.
- How much of the house you open up. A modest extension with a wide structural opening into the existing house can be a bigger job than a large one with a small opening. Steel over a wide span has to be sized, supported, and protected properly.
- The roof. Flat, pitched, tied into the existing roof or standing on its own. This decides both how the extension looks from the garden and how much work happens at the junction with the old house — which is where a badly built extension leaks.
- Glazing. Bifolds, sliders, a lantern or plain windows is often the single biggest material decision in the whole job, and it is worth making it early because it affects the structure above it.
- Access. Whether a machine can reach the back of the house, or every barrow of spoil comes out through your hallway.
How the work actually runs
- Survey and first ideasWe visit, measure up and talk about what the extra space is really for. That conversation changes the plan for the better more often than not.
- Design and drawingsDrawn in-house and developed with you, then worked up into what planning and building control need, with the structural calculations coordinated for the build.
- PermissionsWe tell you whether your address is permitted development or needs a full application, and then we handle it — it is our job, not yours.
- Written quote and programmeA clear written quote and a build programme land before anything is dug, so the cost and the order of work are agreed up front.
- GroundworksClearance, the dig, drainage diverted or rebuilt where it has to be, then foundations poured and inspected before anything goes on top of them.
- ShellWalls up, steels in, floor down, roof on. The extension is made weathertight as its own building before it is joined to yours.
- Breaking throughOnly then does the wall between old and new come out, propped and supported on fresh steel. This is the noisy, dusty part, and doing it late rather than early is what keeps the rest of the house liveable.
- First fix, plaster, second fixHeating, plumbing and electrics; windows and doors; then plaster and screed, second fix, and the kitchen or bathroom if one is going into the new space.
- Finish, sign-off, handoverDecorating and making good inside and out, building control sign-off coordinated as part of the job, then snagging and a proper handover.
Throughout it you can see where things have got to in our app — progress photos, quotes, invoices and messages tied to your actual job — rather than working it out from what you can see through the kitchen window.
What drives the cost
There isn’t one figure we can give you, and it is the same story across all three counties we cover. Cost follows the size of the extension, the ground conditions, the finish you choose and how easy the site is to get to — plus how much of the existing house has to be opened up and re-supported. In detail:
- Footprint and how many storeys you are adding.
- What the dig turns up — ground conditions, trees, drainage, and how deep the foundations have to go because of them.
- The number and width of structural openings between the new space and the old.
- Roof type and glazing spec, which usually move together.
- Whether a kitchen or a bathroom is going into the new space.
- Access for machines and for taking spoil and waste away.
- The level of finish, inside and out.
We don’t publish a generic price list, because a figure with none of the above behind it would be misleading. Every quote is free and no-obligation — tell us about the project and we’ll come and look at the actual house.
How long it takes
It varies with the size of the job and how long planning or building control approval takes; the weather plays a part too, particularly at the groundworks and roof stages. We’ll give you a realistic timeline as part of your quote, and once work starts you can follow progress yourself through the app instead of waiting on a phone call.
Planning permission and building control
Many single-storey rear extensions fall under permitted development. A conservation area, a listed building, a flat, or a house that has already been extended can all take that away, and two-storey work is more likely to need a full application. How close the work sits to a boundary matters as well.
Building regulations apply either way, and they reach further into an extension than most people expect: foundations, structure, damp-proofing, insulation, drainage, ventilation, and the openings formed between the old house and the new one.
We handle the drawings, the permitted-development advice and the application in-house, and coordinate building control sign-off alongside the build. Where the work runs along a shared boundary or an attached neighbour’s wall, party-wall formalities may need settling first. We’ll tell you exactly what your address needs before any work starts.
What we handle ourselves
An extension touches nearly every trade there is, which is exactly where projects normally come apart — one firm waiting on another, and nobody owning the gap. We are a family firm running the whole thing ourselves, with one number to ring.
- Drawings, design and the planning application
- Demolition, site clearance and groundworks
- Foundations, drainage and the structural shell
- Structural openings and steelwork
- Roofing, windows and doors
- Heating, plumbing, electrics and air conditioning
- Plastering, bricklaying, decorating and putting the garden back afterwards
An extension we have built
On a red-brick house in Hemel Hempstead we have built a rear extension from the ground up: foundation brickwork matched to the existing walls, a suspended beam-and-block ground floor laid dead level to the floor inside, and a steel beam let into the back wall to open the house into the new room.
Read the Hemel Hempstead extension case study — the parts you never see once it is finished, photographed on site.
Where we work
We cover Bedfordshire, Hertfordshire and Buckinghamshire. There is a postcode checker on the homepage if you want to know for certain, and it is worth asking even if you fall just outside — we take on the right jobs a bit further out.