The foundations are one of the most important parts of an extension. The design of foundations can vary for each project, dependent on factors such as ground conditions, nearby trees, topography of the site, or existing and proposed drains or sewers.

For domestic extensions, traditional trench-fill or strip concrete foundations are most common. These must be taken down to firm natural ground. Your Building Control Officer will be able to assist you on site to determine the nature of the ground conditions, or the impact of nearby trees and drains, and therefore advise you on the recommended depth or width of foundations.

Where conditions on site are not suitable for traditional foundations, more specialist solutions may be required (e.g. reinforced raft foundations, piled foundations). In these circumstances you will need to employ a qualified structural engineer to provide a design for your foundations.

The ground floor of an extension needs to be structurally adequate and robust, but should also provide protection against damp penetration, and thermal insulation. There are several options for ground floor construction, depending on factors such as site conditions and existing construction, as well as financial and practical preferences.

A ground-bearing concrete floor is a layered construction, with a firm and compacted sub-base of suitable hardcore material, damp proof membrane, layer of insulation, and solid concrete slab at least 100mm thick, finished with a level screed.

In some circumstances, such as where the floor is significantly higher than ground level, or where ground conditions are not suitable for a concrete floor, a suspended floor may be more appropriate. These can be either of timber or concrete construction, with thermal insulation is generally provided between the joists on a timber floor, or above a suspended concrete floor . A void beneath the suspended floor, ventilated to outside air, provides the necessary protection from damp.

The walls of an extension must carry the structural loads of the upper floors and roof, keep the weather out of the extension, and provide thermal insulation. External walls can be either masonry cavity wall construction, or timber frame construction.

Cavity walls are usually of brick or block construction, with an insulated central cavity. The two skins of masonry are held together with steel ties to ensure stability. Where openings are provided for doors and windows, a suitable lintel, generally of steel or concrete, is required over the opening. Damp proofing is provided by a horizontal damp proof course at least 150mm above ground level.

Timber frame construction comprises an insulated structural timber stud wall, with protection from the weather being provided either by an outer skin of brick, or by hung tiles or weatherboarding. This type of construction may need to be engineer designed, and where built adjacent to a boundary with a neighbour, may also require additional layers of fire resisting material to prevent spread of fire.

Where the proposed extension is more than one storey, the upper floors will generally be of timber joists spanning between loadbearing walls. Suitable flooring is provided above the joists, and a plasterboard finish below to form a ceiling. While thermal insulation is not required to internal floors, suitable insulation is required to provide sound resistance between levels.

The roof of an extension is designed to keep out the weather, provide thermal insulation, and to bear the load of the roof coverings, light storage to loft spaces, and occasional build-up of snow. Domestic roofs will be either of flat or pitched roof construction.

Flat roofs can be a relatively simple, practical and economical solution for extensions. They are generally constructed of a timber joists spanning between loadbearing walls, with a plywood or similar roof deck above, finished with roofing felt or other alternative material for weatherproofing. Insulation is either provided between the roof deck and the felting to form a warm deck, or is added between and below the timber joists to form a cold ventilated roof.

Pitched roofs can either be a traditional cut timber roof, or can come as pre-formed engineer designed roof trusses from a specialist manufacturer. While truss roofs are made and delivered to order, a cut roof is usually designed by a architect or engineer, then cut and pitched on site by a carpenter. Most modern roofs are overlaid by a breathable membrane (although traditional roofing felt is still acceptable), and then covered by overlapping tiles or slates for weather resistance. Where a flat ceiling is provided inside, insulation is provided by loft quilt rolled between and over the ceiling joists. For a vaulted roof, insulation may be cut between and below the rafters.

With the exception of warm deck roofs, all roofs will require ventilation between the underside of the roof deck and the insulation, to prevent build-up of damp and condensation. This is provided via breathable roofing felts, and through additional ventilation grilles at the eaves and ridge.

For any extension, drainage for surface water must be provided to discharge rainwater water from the new roof. If the extension includes new kitchens, bathrooms or utility rooms, then new foul drainage runs will also be required. The foul and surface water systems should be kept separate wherever possible, and allow drainage to discharge to a suitable final disposal point. It is worth identifying existing drain locations and planning drainage layouts at the earliest possible stage, as they can have an impact on the design of other elements such as floors and foundations.

All drainage systems must be laid with a suitable fall to allow gravity to carry the water and effluent downstream, and must be provided with adequate ventilation, and ideally in straight runs with access points for rodding at change of directions. All below ground drainage should be laid in suitable granular fill, and capped with concrete where cover is shallow.

Foul water should discharge to a mains public foul sewer, or to a treatment plant or cesspool. Surface water should discharge to a public surface water sewer, adequate soakaway, or natural water course. A surface water connection should only be made to a foul or mixed public sewer where there is no reasonable alternative, and where the expressed permission of the local water authority (e.g. Southern Water) has been obtained.

It is very important to consider fire protection when designing and planning an extension. Domestic fires pose a serious risk to life, and the most dangerous can occur at night when people are asleep. It is important fire safety is considered as early as possible, as it may have a significant impact on the proposed design and layout of your extension.

In order to provide adequate early warning, smoke detection should be installed on each storey of the house, and should be mains powered and interlinked to the relevant British Standards.

As well as early warning, adequate provision should be allowed for means of escape from habitable rooms. This can be via access to a suitable protected escape route, via hallways and landings to a final exit door. Where the ground floor is open plan, or where inner rooms are present, escape from ground and first floor can be via a suitable emergency egress window.

Further advice on the above is readily available, and your Building Control officer will be happy to discuss fire safety measures with you before and during your project.

The Building Regulations require that adequate ventilation be provided to new buildings and extensions. This can generally be achieved easily in domestic extensions through provision of windows with an openable area at least 1/20th the area of the floor in each room, with background ventilation provided by trickle vents fitted to the head of windows and doors.

New windows and doors should achieve minimum standard for thermal performance as required by the Regulations, usually through use of double glazed low-e units.

Any glazing provided in critical areas, such as doors, window panes adjacent to doors, and windows with a cill height below 800mm should be provided with suitably robust toughened glass. Windows above ground floor level, where the bottom of any opening is below 800mm, should also be provided with guarding or restrictors to prevent danger from falling.

It should also be remembered that certain windows may need to be suitable for means of escape in case of fire. Please see guidance above on Fire Safety for further details.

Where the extension includes a new bathroom, WC, kitchen or utility room, it you should also install mechanical extraction fans to ensure removal of excess moisture from the house, and prevent condensation and mould. Your Building Control officer can advise you on the required extraction rates appropriate for each location.

Any new electrical works or heating services installed during extension works should be carried out by a qualified installer, who is registered with the relevant Competent Persons scheme, such as NAPIT or NICEIC for electricians, GASSAFE for gas central heating and boiler installations, OFTEC for oil fired boilers, or HETAS for solid fuel burning stoves. The installer will provided you with an installation and commissioning certificate for the works carried out, a copy of which will need to be provided to Building Control prior to completion of works.

Where any fuel burning appliance is provided inside the house, it is necessary to provide carbon monoxide detection adjacent to the appliance.