Over Garage Extensions

What an over garage extension actually is

A habitable room built above an existing single storey garage, using the garage walls and foundations to carry it, where they are capable of carrying it.

The brief is almost always the same. A three bedroom semi or detached house with an attached or integral garage down one side, and a family that needs a fourth bedroom, a proper office, or an en suite the existing landing cannot accommodate. The garage is already there and already attached. Building on top of it does not touch the garden and does not eat into the driveway.

That is a genuinely good reason to want it. A room above the garage is the only first floor addition that costs nothing at ground level, and where the loft is already converted or too shallow to convert, it is often the only route to another bedroom at all.

It is not a garage conversion, which is a change of use inside the existing ground floor envelope with nothing new above it. It is not a two storey side extension from scratch, which starts with a fresh dig and foundations designed for the load. An over garage extension sits between those two, and where it lands between them is decided entirely by what is holding the garage up.

Single skin walls, Part L, and the room you are left with

A garage wall in one brick skin cannot be the external wall of a bedroom, and both ways of fixing that cost you something.

Plenty of attached garages are built in half brick, a single 102mm skin with no cavity, no insulation and sometimes no damp proof course worth the name. Others are cavity but thin or entirely empty. Either way that wall does not meet Part L as the external wall of a heated room, and it will not be made to by argument.

The first answer is an internal insulated lining, with the vapour control and junction details done properly. It works, it is comparatively cheap, and it eats the room. Allow 100mm to 150mm off each wall you line. A garage with a 2.4 metre internal width comes out at around 2.1 metres of finished room, which takes a single bed comfortably and a double bed grudgingly. Measure that out on the floor before deciding it is fine.

The second is to rebuild the wall in cavity construction off a widened foundation, giving a full performance wall with a proper cavity tray, damp proof course and insulated closers around the openings. It costs more, and if the wall is coming down anyway then the third outcome above is a shorter step than it looked. Settle which of the two you are doing early, because the width of the room is what the whole layout hangs off.

The Party Wall Act, which applies here more often than anywhere else

Garages get built on the boundary because that is where the spare land was, and that puts this job in the Act as a near certainty.

Three parts come into play. Section 1 covers building a new wall on or at the line of junction with the neighbour’s land. Section 2 covers work to an existing party wall, including cutting into it, raising it or building off it. Section 6 covers excavation within three metres of a neighbouring structure where you dig deeper than its foundations, and within six metres where the dig falls below a line drawn at forty five degrees from the bottom of theirs.

Underpinning a garage wall near a boundary triggers section 6 almost automatically, because that is precisely what underpinning is. Raising a wall that is shared or stands on the line of junction triggers section 1 or section 2. On a pair of semis whose garages are built back to back, a very common estate arrangement, you can be in more than one at once.

The notice periods are fixed in law: two months for section 2 work, one month for sections 1 and 6. Your neighbour can consent in writing, which costs nothing and ends the matter, or dissent, in which case surveyors are appointed and an award sets out what may be done and how any damage is put right. The building owner pays the reasonable surveyor costs, and a schedule of condition prepared beforehand protects both sides equally.

Serve early. Party wall runs in parallel with planning, so started at the same time it costs nothing in programme. Left until permission comes through, it adds up to two months to the date you break ground.

Building Regulations, and the fire separation nobody expects

Separate from planning, needed either way, and on this job it is where the awkward details live.

Planning decides whether you may build. Building Regulations decide how it is built, and they apply whether or not planning permission was needed. Here the Approved Documents doing the work are A for structure, B for fire safety, C for damp and the condition of the existing fabric, F for ventilation, K for stairs, guarding and glazing, L for insulation and glazed area.

Part B is the one people have not thought about. Where a garage stays a garage below a habitable room, the construction separating them has to resist fire for a period, normally thirty minutes, applied to the floor between them and to any wall between garage and house. A door from the garage into the house is a fire door, self closing, and there is usually a step up from the garage so spilt fuel cannot run under it. The new bedroom needs an escape window, at least 0.33 square metres of clear openable area with no dimension under 450mm. Existing smoke alarms get extended and interlinked to cover the new room.

Part C earns attention because you are keeping old fabric: garage slabs often have no damp proof membrane and garage walls often have a damp proof course that is bridged or missing, and both become your problem the moment there is a heated room above.

There are two routes through Building Control. Full plans means the drawings and the engineer’s calculations are checked and you have a decision before you start. A building notice means you tell the council you are starting and they inspect as you go. On an over garage job we use full plans without exception, because the whole question is whether the existing structure can take the load, and you want that answered on paper before there is scaffold on your drive. We notify Colchester City Council Building Control, manage the inspections, and you receive a completion certificate at the end.

How long it takes

Roughly three months of paperwork you can overlap, then two to four months on site depending on what the hole said.

The front end is longer than on a single storey job because a planning application is close to unavoidable. Drawings and structural design take three to five weeks, a householder application runs eight weeks from validation, and full plans approval runs alongside it. Party wall notices need one or two months depending on the section, and those periods are set in law. Run in parallel, the realistic front end is about three months.

On site, where the foundations are adequate, an over garage extension is commonly eight to twelve weeks from scaffold up to completion. Underpinning adds two to four weeks and it all falls at the beginning, before the parts you can see. A demolition and rebuild adds more again and should be programmed as a new build on a two storey timetable.

The disruption profile differs from a ground floor extension. A rear extension happens outside the house until the very end. This one arrives at first floor level, in the middle of the sleeping accommodation. Forming the doorway off the landing means a wall out, a temporary partition, and dust in the part of the house you cannot escape from. It is a matter of days rather than weeks, and it is planned with you in advance rather than announced on the morning. Scaffold also takes the driveway out of use for the duration.

How we work

One contract, one contact, and the hole gets dug before the price is firm.

Our team has 25+ years in the trade and comes out to you. The first visit is a survey rather than a sales call: the garage construction, the wall thickness, the roof, the levels between garage slab and house floor, where the boundary runs, what the planning history shows and what you are trying to achieve upstairs.

We will give you an indicative figure at that stage and tell you what it assumes. Before that becomes a firm price we dig the trial hole, because on this job everything else is downstream of it. From there the architectural technician draws it, the structural engineer calculates it, we handle the planning route and the party wall notices, we notify Colchester City Council Building Control, and we run the trades as main contractor.

We carry £2m public liability cover and everything is built to current Building Regulations. We notify Building Control, arrange the inspections and see the job through to the point where they issue your completion certificate. We are based in West Bergholt, about three miles north west of Colchester, and we work across the city and roughly thirty miles around it. The survey and the quotation cost nothing.