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Building Regulations

The conversion of a garage, or part of a garage, into habitable space will normally require approval under the Building Regulations.

As part of the garage coversion, it is likely that the original garage door will be infilled with a new wall and possibly a window or door. As the foundation to the existing garage is not likely to be traditional (it's probably a shallow slab), a new foundation may be needed for the new wall.
The existing foundation may be checked by digging alongside it until it's bottom is reached.

Foundations are required to transmit the load of the building safely to the ground. Therefore, all buildings should have adequate foundations (normally concrete), which will vary from one project to another depending on the circumstances of each case.

These foundations can be cast as deep-fill (filling most of the trench) or shallow-fill (where the minimum thickness to transfer the load to the soil is provided).
There are other types of foundations that may be used if the ground conditions do not make trench fill practicable. It is advisable to contact a structural engineer or speak to building control for further advice.

Factors to be taken into account of when designing a foundation:

Type of soil
The type of soil that the foundation will sit on is important for two reasons:
it should be able to bear the weight (load) of the foundation and the extension - different soils have different load bearing capabilities.
the way it reacts to variations in moisture content (such as in prolonged rainy or dry seasons) can lead to the soil expanding or contracting.
This is a particular issue with some clay soils. These changes mainly occur up to a certain depth (typically about 0.75m) therefore foundations should be made deeper so they are not affected by ground movement (although see "Trees" below). Adjacent structures

It is important to ensure that the excavation for the new foundation does not undermine adjacent structures. In general it is good practice to excavate at least to the same depth as the bottom of the foundation to the adjacent building. If the excavation runs alongside an existing footing then
care will be needed - for example, by excavating and concreting the foundation in shorter sections to avoid undermining a whole length of an adjacent
structure (see also guidance on the Party Wall Act).


Trees will draw moisture from the ground around them and beyond through their root system. As moisture is drawn from the ground it will have a tendency to shrink.
How much the ground will shrink will depend on the following factors:
Type of soil - Clay soils shrink more than other types of soil. Therefore excessive movement of the ground could cause damage to the foundation and the structure it supports.
Size and type of tree - How large a tree or shrub will grow (its mature height), and the tree type will determine how much moisture it generally draws from the ground.
The presence of trees in clay soil areas can mean foundations need to be significantly deeper than might be first expected, although if the trees are far enough away, there may be no impact. Note: If existing trees are removed or significantly reduced in size, all or some of the moisture in the root system will be released over time into the soil and, if the soil is clay for example, could cause swelling of the soil and damage to nearby foundations and structure(s) supported.

Drains and sewers

As the weight (load) from the foundation of a building is transferred to the soil it spreads downwards outside the footprint of the foundation at a typical angle of 45 degrees.
If a drain or sewer is within the area coveredby that 45 degrees area there is a risk that it could be affected by the load from the foundation and possibly crack. Therefore, the
foundation excavation should normally be at least to the same depth as the bottom (invert) of the deepest part of the drain, sewer or its trench.


The existing garage floor is likely to be strong enough for general domestic use, but may need to be upgraded to ensure it is adequate in terms of damp-proofing and thermal insulation.
It may also be desirable to change the level of the floor to match the levels in the existing home. The simplest way to achieve this would be to ugrade the existing concrete floor. Alternatively, if levels permit, an new timber floor could be constructed over the existing concrete floor.

Solid floor

The existing concrete floor can be used as a base, however a new damp proof membrane (DPM) will need to be introduced. DPMs come in solid or liquid form, the latter being a practicable solution for a garage conversion. Manufacturers will be able to advise. A suitable gauge damp proof membrane (DPM) and thermal insulation must be provided.
These can be laid over the sand blinding or on top of the concrete.

Thermal insulation may be required and can be placed on top of the membrane (if a liquid membrane is used care should be taken to ensure the two materials do
not react with each other - a separation layer may be needed). The exact details will vary depending on which products are used.

The floor can be finished with a layer of screed or a timber covering ("floating floor") the exact specification of which will depend on the insulation material used beneath.
A screed is likely to need to be in the order of 75mm thick and should include a reinforcement mesh to prevent it cracking.
Care should be taken to ensure any existing airbricks for the main house are not obstructed by this work. If so, they should be extended through the new floor to external air.

Suspended Timber Floor

The existing floor level to the house may be quite high above ground, and in cases such as this it is more practicable to use timber joists, with a void underneath.
A minimum gap of 150mm should be kept between the existing concrete ground and the underside of the timber. The timber floor joists must be sized correctly
depending on their length. They are then laid across the shortest span from wall to wall with a gap underneath.

An intermediate wall with a small footing may be needed to reduce the span and keep the thickness of the floor joists to a minimum. A damp proof course (DPC)
should be placed on the underside of timber. Insulation is then placed between the joists (thickness required depends on the product used). Air vents should be placed
underneath to provide ventilation to the void and the air should be able to travel from one side of the building to the other.

Contaminated ground
In some areas, the ground could have a certain amount of contamination where gases form. If this is the case then this gas needs to be ventilated and a gas membrane will be required to stop it from entering the building.

Advice on this can be found in the Building Research Establishment (BRE) / Environment Agency report BR 414 ‘Protective measures for housing on gas-contaminated land. 2001’’
For Radon gas there is guidance this in a published report by BRE BR211, ‘Radon: Guidance on protective measures for new dwellings 1999’ Walls below ground level

Depending on whether the foundation has been cast as deep-fill or shallow-fill, there could be a small or large amount of wall construction needed below ground level (substructure), on which the above ground walls (superstructure) will be built.

The principal requirement of the substructure is to ensure adequate support is provided to the superstructure. To remain effective the bricks or blocks and mortar should be resistant to frost and also to sulphates within the ground.


Each new room in a house should have adequate ventilation for general health reasons. The type of room will determine how much ventilation is required.
When inserting a new internal wall care should be taken not to make any other matters, such as ventilation worse. If a new room is being created as a result of the addition of an internal wall then care should also be taken to ensure that the existing room is ventilated adequately. The general rules for ventilating a room are:
Purge - this is achieved by opening the window. The opening should have a typical area of at least 1/20th of the floor area of the room served, unless it is a bathroom which can be any openable size.
Whole Building - this is also known as trickle ventilation which can be incorporated in to the head of the window framework, or by some other means.

The area varies on the type of room:

Bathroom – 4000mm²
All other rooms – 8000mm²
Both of these forms of ventilation are normally required, however alternative approaches to ventilation may also be acceptable, subject to agreement with the Building Control Body.

Mechanical extract fans

Any new kitchen, utility room, bath/shower room or WC with no openable window should be provided with a mechanical extract fan to reduce condensation and remove smells.
The necessary performance of these extract fans is normally measured in litres per second (l/s) as follows:

Kitchen - 30l/s if placed over the hob and 60lt/s if place elsewhere.
Utility room - 30l/s
Bath/shower - 15l/s with a 15 minute overrun (after the light is switched out) if there is no openable window.
WC - 6l/s with overrun.
Alternative rates may be applicable if the ventilation is running continuously.

Sorce: Planning Portal


This is an introductory guide and is not a definitive source of legal information.


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