Planning permission and Building Regulations are two entirely separate legal requirements, and confusing them is one of the most common mistakes homeowners make at the start of an extension project. You can need one without the other, both, or neither — but for the vast majority of single- and two-storey extensions, Building Regulations approval is mandatory. This guide explains what the regulations actually cover and how to navigate the process.
What Building Regulations Cover (and Why They Exist)
Building Regulations set the minimum standards for the design and construction of buildings in England and Wales. They protect the health and safety of occupants, neighbouring properties, and future owners. Unlike planning permission (which is about whether something should be built), Building Regulations are about how it must be built.
The regulations are organised into Approved Documents — one for each subject area. The ones that typically apply to a house extension are:
| Part | Subject | What it means for your extension |
|---|---|---|
| A | Structure | Foundations, walls, beams, lintels, roof loads |
| B | Fire safety | Escape routes, fire doors, smoke alarms |
| C | Site prep and moisture | Damp-proof courses, subfloor ventilation |
| F | Ventilation | Trickle vents, extract fans in kitchens/bathrooms |
| J | Combustion appliances | If you’re installing a stove or boiler |
| L | Conservation of fuel and power | Insulation U-values for walls, roof, floor, and glazing |
| M | Accessibility | Step heights, door widths, sanitary facilities |
| P | Electrical safety | Notifiable electrical work in habitable rooms |
Most extensions also trigger a review under Part Q (security) if new external doors or ground-floor windows are installed, requiring PAS 24:2022-certified hardware.
Do I Need Building Regulations for My Extension?
The main exemption is for qualifying conservatories (Regulation 9 and Schedule 2 of the Building Regulations 2010) where the structure is thermally separated from the main house by an external-quality door, the heating is independently controlled, and the roof and walls are at least 50% and 75% translucent respectively. Remove any of those conditions and the exemption disappears.
Porches of less than 30 m² with no internal door to the house are also exempt, as are certain detached outbuildings under 30 m² (depending on proximity to boundaries and construction type).
For everything else — a rear extension, side return infill, two-storey addition, above-garage conversion — Building Regulations apply.
How to Apply: Full Plans vs Building Notice
There are two main application routes:
Full Plans application: You submit detailed architectural drawings and specifications before work starts. The local authority (or approved inspector) checks the drawings against all relevant Parts and issues a formal approval notice — typically within 5 weeks (or 2 months for complex schemes). Inspectors then visit the site at key stages. On completion, you receive a Completion Certificate. This route is recommended for any project over about £15,000.
Building Notice: You notify the local authority of your intention to build at least 48 hours before starting. No drawings are submitted in advance — the inspector reviews the work on site at each stage and advises you of any issues. Faster to start, but if the inspector requires changes after foundations are poured, the cost of remediation falls on you. Not suitable for extensions near public sewers (which require a Section 80 drain connection notice under the Water Industry Act 1991 in any case).
Application fees are set locally but are typically £200–£450 for a single-storey extension under 60 m² (Building Notice) or £350–£900 (Full Plans). You can also use a private Approved Inspector (now regulated by the Building Safety Regulator since April 2024) — fees are negotiated and often comparable to local authority charges.
Foundations
The Building Control inspector will expect to see the foundation excavation before concrete is poured. The required depth depends on ground conditions and proximity to trees:
- Minimum strip foundation depth in stable clay: 750 mm (Building Regs Approved Document A, Table 10)
- Near trees on clay: can exceed 2,500 mm to get below the zone of seasonal moisture variation — a structural engineer should advise
- Width of strip foundation: minimum 450 mm for a single-storey extension, but typically 600 mm for masonry cavity walls
If you are building close to existing foundations (within 1 m or at a lower level), the inspector may require an engineer’s report confirming the existing footings are adequate.
Structural Requirements
Part A governs the structural integrity of the extension itself and its connection to the existing building. Typical requirements:
- Lintels over openings: minimum end bearing of 150 mm each side; specified by a structural engineer or from standard span tables
- Cavity walls: 100 mm inner leaf (blockwork, min 3.5 N/mm² compressive strength), 100 mm outer leaf (facing brick), 90–100 mm insulation fully filling the cavity to achieve Part L compliance
- Roof structure: engineer-designed flat roofs with structural calculations; for pitched roofs, TRADA span tables are typically used for rafters up to 5.5 m span
- Wall ties: stainless steel, typically at 900 mm horizontal and 450 mm vertical centres (denser near openings and edges)
If you are removing a section of the existing external wall to merge the extension with the house, the structural beam over that opening must be designed by a structural engineer and signed off by Building Control.
Insulation (Part L)
Part L: Conservation of Fuel and Power (2021 edition, in force from June 2022) sets maximum U-values for the building fabric:
| Element | Maximum U-value (W/m²K) |
|---|---|
| External walls | 0.28 |
| Floor | 0.25 |
| Roof (flat or pitched) | 0.16 |
| Windows and doors (whole unit) | 1.40 |
| Rooflights | 1.80 |
You must also demonstrate that the notional heat loss through the extension does not worsen the overall energy performance of the dwelling beyond a set threshold. In practice, modern full-fill cavity insulation (typically 90–100 mm mineral wool or PIR, achieving around 0.20–0.22 W/m²K wall U-value) and a 150 mm insulated flat roof system exceed the minimum requirements comfortably.
Drainage and Waterproofing (Part C and H)
If the extension footprint covers any underground drainage runs, you will need to either divert them or cover them with an access chamber — building over an adopted sewer requires approval from the relevant water company under Section 183 of the Water Industry Act 1991.
Part C requires damp-proof courses at a minimum of 150 mm above external finished ground level and a continuous DPC linked to the existing DPC in the house wall. Suspended timber floors need underfloor void ventilation (minimum 1,500 mm² per metre run of perimeter wall); solid floors need a damp-proof membrane below the insulation.
Fire Safety (Part B)
A single-storey rear extension rarely creates new fire risks, but Part B requirements that typically apply include:
- Smoke alarms in new habitable rooms and in the circulation route between the extension and any existing smoke alarms (Grade D, LD2 minimum, interlinked)
- Fire doors where the extension creates a new inner room or where the escape route passes through the extension to the staircase
- If the extension brings a boundary wall within 1 m of the boundary: maximum permitted unprotected area in that wall is very restricted — check with Building Control before designing large bifold doors in a side wall close to the fence
Electrical Work (Part P)
Any new electrical circuits, consumer unit work, or notifiable electrical work in a bathroom within the extension must be carried out by a Part P-registered electrician (NICEIC, NAPIT, or ELECSA registered) who self-certifies the work. Alternatively, the homeowner can use any electrician but must notify Building Control to arrange a test.
Inspections and Sign-Off
A typical inspection schedule for a single-storey extension:
- Commencement — notify Building Control before work begins
- Foundations — before concrete is poured
- Oversite — DPC level, before floor is laid
- Drains — before backfilling
- Structural frame / roof (where applicable)
- Insulation / pre-plasterboard — so the inspector can check wall and roof insulation
- Completion — final inspection; inspector will check ventilation, fire detection, electrical certificate, and visible finishes
On satisfactory completion, the local authority issues a Completion Certificate (or the approved inspector issues a Final Certificate). Keep this document safely — it is required when selling the property and is checked by solicitors during conveyancing. Without it, your solicitor may require you to take out indemnity insurance.
