If you want to put up small detached buildings such as a garden shed or summerhouse in your garden, building regulations will not normally apply if the floor area of the building is less than 15 square metres and contains NO sleeping accommodation.
If the floor area of the building is between 15 square metres and 30 square metres, you will not normally be required to apply for building regulations approval providing that the building contains NO sleeping accommodation and is either at least one metre from any boundary or it is constructed of substantially non-combustible materials.
This section provides you with general information to help you comply with the Building Regulations when constructing a new outbuilding within the boundaries of an existing property, such as:
- garage or carport
- summerhouse or shed
Building a new garage attached to an existing home would normally need building regulations approval.
Building a new attached carport (open on at least two sides) would not normally require building regulations approval if it is less than 30 square metres in floor area.
Building a detached garage of less than 30 square metres floor area would not normally need building regulations approval if:
- the floor area of the detached garage is less than 15 square metres.
- the floor area of the garage is between 15 square metres and 30 square metres, provided the garage is at least one metre from any boundary, or it is constructed from substantially non-combustible materials.
In many cases, these structures will be exempt from requiring approval under the Building Regulations if they meet certain exemption criteria.
If the Regulations do apply to the building then it must be built to reasonable standards.
The following give an indication of some of the elements normally required to satisfy the requirements of the Regulations when building an outbuilding:
- Walls below ground
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).
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.
Size and construction of new building
The foundation will need to support more weight (load) from a two storey building compared to a single storey. This has a significant factor in determining design, particularly in respect of its depth and width. This is directly related to the bearing capacity of the soil supporting it. The width of the foundation is also governed by the wall thickness.
Generally the topsoil is taken away and good undisturbed ground is found i.e. ground that has not been built on. In some cases there are areas which have previously been backfilled, such as above where drains have been laid or to level a site, which consist generally of soft, mixed soil with foreign objects. The foundation can not be poured until undisturbed ground has been found.
Some properties have been constructed on landfill sites which may require a more extensive form of foundation like piling as the depth of undisturbed ground could be many metres deep. An alternative may be a "raft" foundation. A structural engineer will be able to advise you further.
For health and safety reasons, care should be taken when working in trenches due to the risk of collapse causing potentially serious injury.
Depending whether the foundation for a new detached building has been cast as deep-fill or shallow-fill, there could be a small or large amount of wall construction needed below ground level (referred to as substructure), on which the above ground walls (referred to as superstructure) will be built.
The principal requirement of the substructure is to ensure adequate support is provided to the superstructure. The substructure (bricks or blocks and mortar) should be effective and be resistant to frost and also to sulphates within the ground.
A floor will need to provide for one or more of the following:
- Structural support of the room’s contents and users and the weight of the floor itself; and
If the floor is a ground floor, provide resistance to:
- Ground moisture; and
- Heat loss (thermal insulation)
There are three general types of ground floor construction:
- Solid, ground bearing, concrete floor
- Suspended Timber Floor
- Suspended Concrete floor
A typical floor for a garage generally consists of hardcore, sand blinding, DPM (damp proof membrane) and concrete. It is considered to be good practice to place reinforced mesh within the concrete as this can reduce the chances of cracking from the load (weight) of a vehicle. The floor will not normally need to be insulated.
Storage buildings, annexes and summer houses
Floors in these types of building can take one of the three general types outlined above. The exact specification for the floor will depend on how the building is to be used and whether it is to be heated. More information on each floor type can be found in the section on floors for extensions.
This is an introductory guide and is not a definitive source of legal information.
This guidance relates to the planning regime for England. Policy in Wales may differ. If in doubt contact your Local Planning Authority.