What Building Regulations are Required for an Extension?

Extension on Home

If you have asked yourself “what building regulations are required for my home extension?” it can be difficult to know where to start. The first step is to understand what exactly building regulations are, and why they’re necessary.

What are building regulations?

Building regulations come from the Building Act 1984. From that act, the Building Regulations 2010 were formulated. Those regulations are fairly broad, however, as they have to cover a number of builds, so clarity comes in the form of Approved Documents, such as Part A covering Structure, Part B covering Fire Safety, etc. These regulations are a series of checks and requirements to ensure that all new builds and extensions conform to uniform building standards. Your extension will likely concern the following elements of the regulations:

  • Roof
  • Internal and external walls
  • Doors, windows, and rooflights
  • Electrical work
  • Drainage
  • Kitchens/bathrooms

With that in mind, let’s discuss these elements in more depth.


Your extension’s roof has to be properly insulated, weather-resistant, damp proof, ventilated, and it should be structurally sound. You’ll likely be using a flat or pitched roof for your extension. If you’re using a flat roof, it’ll often be felt-covered, with minimal fall for water run-off. Pitched roofs, however, are usually tiled. Whichever roof type you choose, it has to be watertight and weather resistant. Consider fire safety too if you’re near to neighbours. The roof will also have to be properly insulated, whether it’s via a warm roof or cold roof system. There will need to be appropriate ventilation too. Structural stability from appropriate roof joists is also a consideration, determined by your structural engineer.


Internal and external walls are monitored to ensure thermal efficiency, structural stability, and weather resistance. You can choose solid or cavity walls for external use, with cavity walls becoming increasingly common. There will need to be appropriate damp-proofing to ensure no water ingress, which is done through the use of an appropriate damp-proof course tiled in with a damp-proof membrane. There will usually be an opening to your extension, either at the rear or side of the house, which should be supported by a steel beam loading onto the brickwork on either side of it. As part of the checks, these will be monitored for installation in accordance with the approved designs.


Lintels will be installed above windows and doors to provide appropriate load-bearing support from above. These need to be installed with the standards laid out in the relevant Approved Documents, and as per the design of your structural engineer. Any glazing that’s installed needs to meet the minimum u-values, as found in the Approved Documents. Usually, manufacturers will have ensured their products meet these standards, and provide supporting data and guarantees to prove it. Any openings for windows, roof lights, or doors must be watertight, insulated, airtight, and not compromise structural integrity.

Electrical installations

If you are, or you’re using, an electrician which is properly registered with an appropriate competent person scheme, they can self-certify any work done without requirement for further sign-off by building control. If they are not registered with any such scheme, building control will have to sign-off. In the case of most extensions, a re-wire of the property is required or considerable modification and alteration of existing circuitry is necessary. These are considered notifiable changes, which means that building control must be informed and allowed to inspect as they deem necessary.

Drainage/waste disposal

You will find appropriate details regarding drainage and waste disposal obligations in Approved Document H. Drainage is split into two categories – foul drainage and surface water. Foul drainage will carry water from toilets, sinks, washing machines, and dishwashers, through relevant pipework down into the sewers. Surface water drainage relates to water from rain and snow which falls onto roof surfaces, before travelling along guttering and downpipes. This can run either into a dedicated surface water draining hole or down into the sewer with foul water. When building over or inside three metres of a public sewer, you’ll need written agreement from the sewer operator. For this, you’ll often have to submit detailed drawings to ensure they’re able to accurately assess the risk of damage to the said sewer. Remember a single sewer system serves multiple properties, usually, so any damage could have a serious knock-on effect for your neighbours. Drainage is a serious consideration of all extension projects. Be sure to consider drainage options through the development of your plans, to minimise the need to alter them later on.


As per Approved Document F, your largest obligation with kitchens and bathrooms in an extension is appropriate ventilation.

What if you don’t seek building regulation approval?

If work is carried out and it’s found not to comply with relevant building regulations, there are several possible outcomes. The first is that you risk being saddled with a large fine. There is no cap on the fine that you can be charged, so it depends on the nature of your build and how far outside building regulations you have strayed. You will be then tasked with rectifying the wrongs, in line with the inspector’s report. The second, if you do not choose to rectify these issues yourself, is the local authority may rectify those issues on your behalf and then reclaim the costs from you. When you come to sell the property, a completion certificate from building regulations is requested to ensure the extension complies. Should any issues be found in the inspection, you will have to rectify them before being issued a completion certificate and being able to push forward with the sale. Alternatively, you can list your property with the work still needing doing, and the buyer can leverage the cost of the work against the asking price.

A brief conclusion

To conclude, building regulations will play an integral part in the way you approach the design and construction of your extension. The easiest path forward is to ensure your extension complies from the outset with all relevant building regulations before construction begins, to avoid costly rectification later on. Only ever use approved builders, electricians, and tradesmen who are aware of, and work within, the building regulations for your peace of mind.