Building Regulations for Extensions: The Complete Guide

An extension touches multiple parts of the building regulations. The Approved Documents that most commonly apply are:

• Part A - Structure: The extension must not overload existing foundations, lintels must be correctly sized over any new openings, and any new structural elements must be designed to carry the loads placed on them.
• Part B - Fire Safety: If the extension creates a new inner room (one from which escape is only possible through another room), escape windows or a suitable alarm system may be required. Cavity barriers and fire stopping must be provided at the junction with the existing building.
• Part C - Moisture Resistance: Walls, floors, and roofs must resist moisture penetration. The damp proof course (DPC) must be correctly installed and linked to the existing DPC in the existing walls.
• Part F - Ventilation: Habitable rooms require both background ventilation (typically trickle vents in windows) and purge ventilation (openable windows of at least 1/20th of the floor area).
• Part H - Drainage: Any new soil or waste pipes must connect to the existing drainage system, and the connection method must comply with Part H. Surface water from the new roof must be managed separately from foul drainage.
• Part L - Conservation of Fuel and Power: This is often the most demanding part for extensions. New and replacement elements must meet minimum U-value requirements (see below).
• Part P - Electrical Safety: If the extension includes new electrical circuits or significant alterations to existing ones, the work must be carried out by a Part P registered electrician or notified to building control.

The 2021 edition of Approved Document L (which came into force in June 2022) sets out the minimum thermal performance standards for new extensions. The key U-values you must achieve are:

• Walls: 0.18 W/m2K
• Roof: 0.15 W/m2K
• Ground floor: 0.18 W/m2K
• Windows, roof windows and rooflights: 1.4 W/m2K (whole window, not just the glazing)
• External doors: 1.4 W/m2K (solid doors: 1.0 W/m2K)

These are limiting fabric values - individual elements must not exceed these figures. Where existing elements (such as a party wall) are already below standard, you are not normally required to upgrade them, but you must not make them worse.

There is also an overall energy efficiency calculation requirement for extensions over a certain size: if the extension is more than 25% of the existing floor area (or more than 1,000m2), a SAP or notional building assessment may be required.

Foundations are the first inspection stage and one of the most important. Your building control officer will inspect the excavation before any concrete is poured. Key requirements include:

• Foundation depth is typically a minimum of 1 metre in clay soils, but may need to be deeper depending on soil type, nearby trees, and frost depth
• Foundation width must be sufficient for the load of the new walls above
• A damp proof course must be installed at least 150mm above external ground level and must be connected to the DPC in the existing wall
• A damp proof membrane (DPM) must be installed under any concrete ground floor slab and linked to the wall DPC to prevent ground moisture rising

Structural lintels are required over every opening in a new masonry wall - doors, windows, and internal doorways through into the existing building. Lintels must be sized by a structural engineer or in accordance with span tables in Approved Document A. Standard steel boot lintels are most common; in cavity walls the lintel must bridge both leaves and include a thermal break to prevent cold bridging.

There are two main routes for building control approval for an extension:

• Full Plans: You submit detailed drawings and specifications before work starts. The building control body checks and approves the plans, then carries out inspections during construction. If the plans are approved, you have written confirmation that your design complies - which is valuable if problems arise later or when you sell.
• Building Notice: You give 48 hours notice before starting work, without submitting plans. Inspections happen during construction but there is no plan approval upfront. If non-compliant work is done and subsequently covered up, it is harder to resolve.

For extensions, Full Plans is strongly recommended. The additional cost of preparing plans is modest compared to the risk of discovering a compliance problem after the walls are up. Mortgage lenders and buyers solicitors will also be more comfortable seeing a Full Plans approval and completion certificate than a Building Notice completion.

You can use either a local authority building control (LABC) officer or a private approved inspector (now called a Registered Building Control Approver, or RBCA) for either route.

Whether you use Full Plans or Building Notice, your building control officer will carry out a series of inspections during construction. The typical inspection stages for an extension are:

1. Commencement: Notify building control before work starts
2. Foundation excavation: Before concrete is poured - the officer checks depth, width, and ground conditions
3. DPC level: Before the cavity is filled above DPC level - checks the damp proof course and oversite concrete (the ground-floor slab)
4. Structural frame / wall plate: Before the roof structure is installed - checks lintels, padstones, and wall ties in cavity construction
5. Roof structure and insulation: Checks rafter sizing, insulation installation, and ventilation to the roof void
6. Drainage: Before backfilling any new drain runs - the officer may require a water test
7. Insulation: Before internal linings are fixed - checks wall, floor, and roof insulation continuity and thickness
8. Final inspection: Once all work is complete - checks electrical installation certificates, ventilation, glazing, and overall compliance

You must give advance notice before covering any stage. Covering work without inspection can result in the officer requiring it to be exposed again.

Extensions regularly require new drainage connections, whether for a new WC, utility room, or simply to manage roof rainwater. The key requirements under Approved Document H are:

• Foul and surface water must be kept separate where a separate drainage system exists. You cannot connect a downpipe to the foul drain.
• New drain runs must be laid at a minimum gradient of 1:40 for 100mm pipes (ideally 1:80) to ensure self-cleansing flow
• Access points (rodding eyes or inspection chambers) must be provided at every change of direction and at regular intervals
• If you are connecting to a public sewer, you may need to notify your water company (a Section 106 connection application) for connections to the adopted sewer
• Surface water should ideally drain to a soakaway, permeable surface, or watercourse - not to the public foul sewer

Building regulations apply to most construction work, but there are limited exemptions. Work that is exempt from building regulations includes:

• Like-for-like repairs and maintenance - replacing a window with an identical window (though replacement with new glazing in a new opening does require compliance)
• Small detached outbuildings under 15m2 floor area (covered in our outbuilding guide)
• Conservatories under 30m2 that are thermally separated from the main house
• Porches at ground level under 30m2

Note that even exempt work may still require planning permission. And even if building regulations do not strictly apply, it is almost always worth building to at least the spirit of the regulations for safety, comfort, and future saleability.

Once your building control officer is satisfied that all work complies, they will issue a completion certificate. This is a critical document - keep it safe.

You will need the completion certificate when you sell the property. Buyers solicitors will ask for it as part of conveyancing, and without it you may need to take out indemnity insurance, which can delay or complicate a sale.

If you used a local authority building control officer, the completion certificate is issued by the council. If you used a private Registered Building Control Approver, they issue a final certificate. Both are equally valid.

If work was done in the past without a completion certificate, you have two options: apply for a retrospective inspection (the officer may ask for some finishes to be opened up), or take out indemnity insurance. Speak to your solicitor about which route is appropriate.