Extending above an attached garage: what the council asks

An extension above a garage needs planning permission in almost every case, so the question is not whether to apply but what the application has to prove. Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015 allows an extension beyond a side wall only where it is single storey, which puts a first floor over an attached garage outside permitted development before any design question arises. The second obstacle is the garage itself. Most attached garages were built to carry a light roof and nothing else, and the foundations are usually the part that fails the test.

Why the existing garage rarely takes the load

A garage built with the house in the 1960s or 1970s was designed for a car, a freezer and a roof. The side walls are often a single skin of brick or 100mm blockwork with no cavity, the floor slab is unreinforced, and the foundation under the side wall may be a shallow strip only a few hundred millimetres deep. Adding a bedroom above puts a new floor load, a new roof load and a different wind load onto that wall.

You will not know what is there until someone digs a trial pit. That is the single most useful thing to do before paying for drawings. A pit at the corner of the garage, dug to expose the base of the foundation, tells your engineer the depth, the width and the soil it sits on.

Where the foundation is too shallow or too narrow, the usual answers are underpinning in short bays, a new foundation inside or outside the existing line, or demolition and rebuild. Rebuild often costs less once scaffolding, temporary propping and the cost of working around a structure you have to keep are added up. And it removes the risk of discovering something worse halfway through.

Foundations, lintels and what the survey should cover

A structural survey for this kind of job is not a homebuyer report. It needs to say, element by element, what the existing garage can carry and what has to change. The parts that matter are the foundation depth and bearing, the wall construction and its slenderness, the lintel over the garage opening, the junction between the garage wall and the main house wall, and the slab if it is to be reused.

The lintel over the door opening is a recurring problem. A precast concrete or steel lintel sized for a garage roof will rarely take a floor and a masonry wall above, and replacing it means propping the whole opening. If the proposed first floor includes a window directly over the garage door, the engineer has to design a single beam carrying both the floor and the wall above the opening.

Soil type changes the answer too. On shrinkable clay, the foundation depth needed is greater, and nearby trees push it deeper again. The British Geological Survey mapping is a reasonable first check on what the ground is likely to be, but it does not replace a trial pit on your plot.

Where permitted development stops

Class A permits extensions to a house within a set of limits. The ones that bite here are straightforward:

• An extension beyond a wall forming a side elevation must be single storey, with a maximum height of 4 metres and a width no more than half the width of the original dwellinghouse.
• Where any part of the extension is within 2 metres of a boundary, the eaves height must not exceed 3 metres.
• On article 2(3) land, which includes conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, side extensions beyond a side wall are not permitted at all.

A first floor over the garage is, by definition, an extension beyond a side wall that is not single storey. So the route is a householder planning application. An Article 4 direction can remove Class A rights entirely in some streets, and a previous extension counts against the limits because they are measured against the original dwellinghouse as built or as it stood on 1 July 1948.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works would be lawful without permission.

Setting the first floor back from the front

Most councils that publish a householder design guide ask for a first floor over a garage to be set back from the front elevation and to sit under a subordinate roof. The reasons are visual rather than technical. A flush first floor turns a two bay house into a wide box, loses the modelling of the front elevation, and reads as a second house grafted onto the first.

A set back of around half a metre to a metre, with the new ridge dropped below the main ridge, usually resolves it. The set back also gives you somewhere to land the valley gutter and keeps the new roof clear of the existing verge.

The depth of the set back is a matter of local guidance rather than national rule, so read the design guide your council has adopted before you commit to a layout. Where no guide exists, the adopted local plan will still carry a design policy, and the National Design Guide sets out the ten characteristics officers tend to work from.

An illustrative example

Take a 1970s semi with a 2.6 metre wide attached garage set flush with the front wall. The owner wants a 10 m² bedroom above. The engineer's trial pit finds a 450mm deep strip foundation on sandy clay, too shallow for the new load, so the garage is rebuilt on a 1 metre foundation using the same footprint. The first floor is set back 700mm, the ridge drops 600mm below the main ridge, and the front window is positioned to line through with the existing first floor opening. This example is illustrative, not a quoted case.

The street elevation of a pair

Semi-detached and terraced houses were designed as compositions. Extending one half of a pair above the garage is the single most visible change you can make to a street of 1930s or 1960s semis, and it is the reason this application type attracts officer resistance more often than a rear extension does.

Two things help. The first is matching materials properly, which means checking the brick for colour range and bond rather than specifying a generic facing brick. The second is keeping the window proportions and head heights of the host dwelling, because mismatched head heights are what makes an extension look bolted on even when the brick is right.

If the neighbouring half of the pair has already extended over its garage, say so in the application and show both halves on the street elevation drawing. A symmetrical outcome is easier for an officer to support than an isolated one. Where cladding or render is proposed to tie the two together, the rules on cladding and render are worth checking first, because those materials are restricted on article 2(3) land.

Parking, and losing the garage you had

Councils rarely refuse a garage conversion on parking grounds alone, but they do ask the question. Many adopted parking standards set a minimum internal dimension for a garage to count as a parking space, and plenty of older garages are below it, which means the space was never counted in the first place. Check the standard in your local plan or parking supplementary planning document before assuming you are losing a space.

Where the garage does count, show the replacement on the block plan. A driveway length of about 5 metres clear of the back of the footway is the usual expectation for one car, and tandem spaces on a shared drive are harder to argue. Bin and cycle storage has to go somewhere once the garage has gone, so put it on the drawing rather than leaving the officer to ask.

What to submit

A householder application for this work normally needs a location plan at 1:1250 with the site edged red, a block plan, existing and proposed floor plans and elevations at 1:100 or 1:50, and a section through the extension showing the set back and ridge heights. The application fee is set by the national fee regulations and indexed annually on 1 April, so check the current figure rather than an older quote.

A short Planning Statement is not a validation requirement for householder applications, but it is the place to deal with the set back, the materials and the parking in your own words rather than leaving the officer to infer your reasoning. A Design and Access Statement is only required by article 9 of the Development Management Procedure Order 2015 for major development, for listed building consent, and for certain development in a conservation area or World Heritage Site, so most garage schemes will not need one. Front extensions face a different set of objections, and the reasons they are harder than rear extensions apply to a flush first floor too.

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