Garage extension cost: two different jobs sold under one name

How garage extension cost works

1. Settle whether it is permitted development. A great many garage conversion are permitted development, which means no planning application, but the limits on depth, height, materials and how much of the plot is already built on are specific and a designer should read them against your own house before anybody prices anything.
2. Get drawings and a structural design. Existing and proposed drawings, then a structural engineer's calculations for whatever is being opened up or carried. On a garage conversion this is a small share of the total and it is what turns an idea into something three builders can price identically.
3. Serve the party wall notices in time. Work on or near a shared wall, or excavation within three or six metres of a neighbour's foundations, engages the Party Wall etc. Act and the notice periods are set in months rather than weeks. Serving late is the commonest reason a start date slips.
4. Price it against a specification, not a sketch. Ask each builder to price the same written specification, with provisional sums shown as provisional sums. Two quotes that differ by a third almost always differ in what is included: the kitchen, the flooring, the decoration and the drainage diversion are the usual absentees.
5. Agree stages, retention and building control. A staged payment schedule against completed work, a small retention until sign-off, and a clear statement of who is notifying building control. The completion certificate is what a future buyer's solicitor will ask for.

Insurance and paperwork

Common questions

What does a garage conversion involve?

Insulating and lining the walls and floor, replacing the garage door with a wall and window, and bringing the floor level and damp proofing up to habitable standards. The existing slab is often the awkward part, because it may sit low and may have no insulation.

Does a garage conversion need building regulations approval?

Yes. Changing a garage into a habitable room is a material change of use, with requirements for insulation, ventilation, damp proofing and fire safety, and it is notifiable work.

Will I lose parking rights?

You may lose an off street space and, in some areas, that matters to a planning officer or to a future buyer. Where a dropped kerb exists it should stay usable, and where parking is at a premium the trade off is worth thinking about before the door goes.

What does above garage extension cost depend on?

On it being a new structure rather than a conversion. Building over a garage needs existing and proposed drawings and a structural engineer's calculations for whatever is being carried; that design is a small share of the total, and it is what lets three builders price the same job identically.

What sets garage extension cost UK-wide?

Which of the two jobs sold under the name you are pricing. Converting an existing garage reuses the slab and the walls and is usually the cheapest way to gain floor area in a house, while building a new garage, or building over one, is a new structure with new foundations, and the two cost very different money.

Registered contractors by region

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