Home extensions

Home extensions

Generally, extensions of properties require approval under the Building Regulations (BR). The following sections give an indication of some of the elements normally required to satisfy the requirements of the regulations.

For more information, see our 'Building regulations' section.


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.

External walls

If you want to re-render or replace timber cladding to external walls, BR may apply depending on the extent of the work.

Where 25 per cent or more of an external wall is re-rendered, re-clad, re-plastered or re-lined internally or where 25 per cent or more of the external leaf of a wall is rebuilt, the BR would normally apply and the thermal insulation would normally have to be improved.

If you want to insert insulation into a cavity wall, the appropriate requirements will be applied to ensure the insulation material is suitable, and that in the case of some foam insulation, the risk of formaldehyde gas emission is assessed.


If you want to repair or replace an existing drainpipe, manhole or associated fitting, approval under BR is not normally required if the replacement is like for like.

However, putting in a new drainpipe, manhole or associated fitting will normally require BR approval. You will also need BR approval for new rainwater or foul drains inside as well as outside the building. The BR also apply to all non-mains foul sewerage arrangements (such as septic tanks), including their outlets and drainage fields.


A floor will need to provide for the structural support of the room's contents and the weight of the floor itself. If the floor is a ground floor, it must also provide resistance to ground moisture and heat loss.

There are three general types of ground floor construction:

  • Solid, ground bearing, concrete floor
  • Suspended timber floor
  • Suspended concrete floor


New roofs

The main function of the roof is to give shelter from the weather. The roof should be able to support loads from various sources:

  • Materials: This would include tiles, battens, felt, insulation etc.
  • Weather: e.g. wind, snow and rain.
  • Maintenance: A person needing to access the roof in order to make repairs.

There are two styles of roof that are generally used:

  • Pitch roof - This is where tiles or slates are used and a void is usually created underneath
  • Flat roof - This usually consists of felting which has a slight fall to allow rain water to drain off

To enable compliance with the requirements of the BR, full details of the roof will be needed, including the dimensions and performance properties of the materials used.

Existing roofs

If you want to carry out repairs on or re-cover less than 25 per cent of the area of an existing pitch or flat roof, you will not normally need to submit a Building Control application. You will need approval, however, if:

  • You carry out structural alterations
  • The performance of the new covering will be significantly different to that of the existing covering in the event of a fire
  • You are replacing/ repairing more than 25 per cent of the roof area, in which case, the roof thermal insulation would normally have to be improved


All electrical systems will need to comply with the technical and safety requirements in the BR. You should use an installer who is registered with a Competent Person Scheme to seek approval from a Building Control body. This is true for most work. However, you do not need to tell them about repairs, replacements and maintenance work or extra power points or lighting points or other alterations to existing circuits (except in a kitchen, bathroom or outdoors).

For more information about the Competent Person Scheme, see our section 'Approval for building works'.

For more information on the BR relating to electrical work, see our 'Maintenance and renovation work' section under the 'Electrics' subheading.

Doors and windows

All replacement glazing and doors need to comply with the BR. The BR apply to thermal performance and other areas such as safety, air supply, means of escape and ventilation.

For more information, see our 'Maintenance and renovation work' section under the 'Replacement doors and windows' subheading.

Providing ventilation in an extension

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, care should also be taken to ensure that the existing room is ventilated adequately.

Mechanical extract fans

Any new kitchen, utility room, bath/shower room or WC with no ventilation should be provided with a mechanical extraction fan to reduce condensation and remove smells.

Energy efficiency

Any extension project will need to take the energy efficiency of the end product into account. For more information, see our 'Energy-efficient homes' section.

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