Extensions and Garage Conversions

Extensions

Planning Permission

An extension or addition to your house is considered to be permitted development, not requiring an application for planning permission, subject to the following limits and conditions:

• No more than half the area of land around the "original house"* would be covered by additions or other buildings.
• No extension forward of the principal elevation or side elevation fronting a highway.
• No extension to be higher than the highest part of the roof.
• Single-storey rear extension must not extend beyond the rear wall of the original house* by more than three metres if an attached house or by four metres if a detached house.
• Maximum height of a single-storey rear extension of four metres.
• Extensions of more than one storey must not extend beyond the rear wall of the original house* by more than three metres.
• Maximum eaves height of an extension within two metres of the boundary of three metres.
• Maximum eaves and ridge height of extension no higher than existing house.
• Side extensions to be single storey with maximum height of four metres and width no more than half that of the original house.
• Two-storey extensions no closer than seven metres to rear boundary.
• Roof pitch of extensions higher than one storey to match existing house.
• Materials to be similar in appearance to the existing house.
• No verandas, balconies or raised platforms.
• Upper-floor, side-facing windows to be obscure-glazed; any opening to be 1.7m above the floor.
• On designated land* no permitted development for rear extensions of more than one storey.
• On designated land no cladding of the exterior

Building Regulations

Most extensions of properties require approval under the Building Regulations.

There are a number of classes of new buildings or extensions of existing buildings that do not need Building Regulations approval, i.e. are exempt from the Regulations.

The following pages give an indication of some of the elements normally required to satisfy the requirements of the Regulations when building an extension.

Flooring

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 floor

A typical way of constructing a solid floor would be to provide a base of hardcore with sand blinding, with a layer of concrete over that. To ensure a level finish to the floor, a layer of screed is added over the top of the concrete, which consists of sand and cement. 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.

The DPM should be lapped on to the damp proof course in the external walls and, if relevant, internal walls around the floor. Thicknesses of the various parts of the floor will depend on ground conditions and the order in which they are laid. If the existing house has air bricks, ventilating existing floor voids for example, then ducting should be provided to allow air through the solid floor and into the void under the existing house. Air bricks are then placed in the new wall.

Suspended Timber Floor

As a requirement of the Building Regulations the structure should be protected against the growth of weeds and other plant-life.  The ground should have a layer of concrete poured across and there should be a ventilated gap of at least 150mm between the underside of the timbers and the concrete, to prevent moisture gathering and affecting the condition of the joists. The timber floor joists should be sized correctly depending on their span (length between supports) and are normally laid across the shortest span from wall to wall with a gap underneath. 

An intermediate wall with a small foundation 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 between the timber and the wall. Insulation is then placed between the joists (thickness 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.

Suspended Concrete floor

This construction is similar to the timber floor above, but uses either pre-cast concrete planks or small pre-cast concrete beams with concrete blocks laid between the beams. They can normally span greater distances than timber joists.  Ventilation is required in the same way as a suspended timber floor.  The manufacturers may well work out the size of the concrete beams and provide the structural calculations.  Otherwise a structural engineer can also provide this service.

Contaminated ground and Radon

In some areas, the ground could have a certain amount of contamination where gases form, such as from landfill sites. Radon is a naturally occurring gas which is found in certain areas of the country. These gases need to be ventilated and a gas membrane will be required to stop them from entering the building.

Foundations

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:

1. it should be able to bear the weight (load) of the foundation and the extension - different soils have different load bearing capabilities.
2. 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.

Trees

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 covered by 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.

Ground condition

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.

Landfill sites

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.

Walls below ground level

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.

Ventilation

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.

Energy Efficiency

There follows a number of different considerations around the subject of energy efficiency, including whether or not an application for Building Regulations approval might be required for specific projects.

Solar Panels

If you wish to add a solar panel to the roof of your home, Building Regulations approval is likely to be needed. The adequacy of the existing roof to carry the load (weight) from the panel will need to be checked and proven. Some strengthening work may be needed. Also as, for example, roof tiles will be removed or omitted to locate and fix the panel(s) the reinstatement should ensure the roof has adequate weather resistance.

Internal Lights

Fixed internal lighting will need to have reasonable provision made to obtain the benefits of efficient electric lighting whenever:

• A dwelling is extended
• A new dwelling is created from a material change of use
• An existing lighting system is being replaced as part of re-wiring works

A way of making your internal lighting more energy efficient is to provide lighting fittings (including lamp, control gear and appropriate housing, reflector, shade or diffuser or other device for controlling the output light) that only take lamps having a luminous efficacy greater then 40 lumens per circuit watt.

The type of light fittings that would meet the above requirements, are fluorescent and compact fluorescent light fittings. Fittings for GLS tungsten lamps with bayonet cap or Edison screw bases, or tungsten halogen lamps would not.

A general way of meeting these requirements is to provide one fitting for every:

• 25m2 of dwelling floor area (excluding garages); or
• one per four fixed light fittings

Lighting fittings in less used areas such as cupboards and other storage areas would not count towards a fitting. If constructing an extension, it may be more appropriate to install the energy efficient light fitting in a location that is not part of the building work. e.g. to replace the fitting in the hall or landing when creating a new room - depending on the likely extent of use of the new room light compared to the hall or landing.

Insulation in a loft

Installing insulation to your loft area requires an application for approval under the Building Regulations. Care should be taken not to block any ventilation at the edges (eaves).

Re-Cycling Water Tanks

These are tanks that recycle surface water (rainwater) that is collected from the roof and ground and so that it can then be re-used within the house for other uses such as toilet flushing or general washing. These tanks are generally placed underground in the rear garden if there is space. An application under the Building Regulations is required to check the new drainage system running to the tank.

Radiators

Any new radiator installed will require a thermostatic radiator valve (TRV) to be installed. A TRV gives better control over the individual room temperatures. It is also encouraged to place TRV's on existing radiators.

Pipe Lagging

This does not normally require Building Regulations approval.

Structural Opening

Once an extension has been made weather-proof an opening is normally made through the existing external walls.

This can be achieved by removing any existing French doors, patio or window openings and, providing the span of and loading on the existing lintel over the existing doors or windows is not increased, further support is not normally needed.

If a new, or wider, opening is to be formed, the remaining wall above the new opening will need to be supported, typically by installing one, or more usually, two new, properly designed, beams.  Any new beam should normally have at least 150mm bearing (overlap onto the existing wall) on each side of the opening and the existing wall beneath the bearings are likely to need to be strengthened to prevent crushing of them.   This may require the installation of an area of dense concrete (cast in-situ or pre-cast), known as a padstones to spread the load. The size of padstones will vary depending on the circumstances of the case in hand.

Fire Safety

If the beam is steel then it should normally be protected against fire so that it will have 30 minutes resistance to fire (if measured in a standard test). There are different ways that this may be achieved, but the most common is the use of two or more layers of properly fixed plasterboard - the thickness of which will depend on the manufacturer's specification. 

If an exposed timber beam is preferred then a calculation is generally required to demonstrate how much inherent fire resistance it has - dependent on it's size and species of timber. A concrete beam, which would normally have steel reinforcement inside it, generally has adequate fire resistance properties, providing the steel inside is adequately covered by the concrete.