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Two Storey Extension Building Regulations: Escape Windows, Structure, the Numbers to Hit and the 7m Planning Rule
Two storey extension building regulations cover far more than the ground floor version. The second storey means bigger foundations and steels, first-floor joists tied into the house, fire escape provision for any new bedrooms, often a new bathroom with drainage and extraction, and a staircase that may now serve more rooms. Planning is stricter too: permitted development allows a two storey rear extension only within tight limits, and a two storey side extension never. This page sets out what building control checks, the numbers your design must hit, how approval works, and where the planning and party wall rules overlap.
Building regulations: always needed for a two storey extension.
Fire: new first-floor bedrooms need an emergency escape window, unless they open onto a protected stair; smoke alarms on each storey.
Insulation targets: walls 0.18, floor 0.18, roof 0.15, windows 1.4 W/m²K; glazing capped at 25% of the new floor area plus any openings covered.
Planning: permitted development allows a two storey rear extension up to 3m deep and at least 7m from the rear boundary.
Which regulations apply
The Building Regulations 2010 are split into Parts, each explained in an Approved Document on GOV.UK. On a two storey extension, these are the ones building control will check:
| Part | What it covers on a two storey extension |
|---|---|
| A, Structure | Foundations for two storeys of load, steels over the openings on both floors, first-floor joists, roof structure, and the connection to the existing walls. |
| B, Fire safety | Escape windows or a protected stair for upper-floor rooms, smoke alarms, fire resistance near the boundary. |
| C, Moisture | Damp-proof course and membrane tied into the house; cavity trays where the new roof meets the old wall. |
| E, Sound | Only relevant where the work affects a wall or floor shared with a neighbour, or creates a new dwelling. |
| F, Ventilation | Purge ventilation through openable windows, background ventilation, extract fans in any new bathroom or kitchen. |
| H, Drainage | A new soil stack and drains for an upstairs bathroom, rainwater, and work near a public sewer. |
| K, Protection from falling | Stairs, guarding at low windows, and safety glass in critical locations. |
| L, Energy | U-values, the 25% glazing limit, and heating controls if the system is extended. |
| P, Electrical safety | New circuits on both floors, done by a registered electrician or inspected. |
Part O, on overheating, applies to new homes rather than extensions to existing ones.
Structure: where the cost and the calculations go
A second storey roughly doubles the load on the new foundations, so they are usually deeper and wider than for a single storey, and deeper again on London clay or near trees, where building control may ask for a soil investigation or follow NHBC tree guidance. The openings where the extension joins the house typically need steel beams on both floors, sitting on padstones or steel columns, and the first-floor joists must be sized and fixed to the existing structure.
Building control will want structural calculations for all of this, normally prepared by a structural engineer, before the relevant stage is built. Check the existing walls too: in older London houses the rear wall may be thinner or shallower-founded than you expect, which is discovered far more cheaply at design stage than with the wall opened up.
Fire safety: escape, alarms and the boundary
For a typical two storey house, with no floor more than 4.5m above ground, Approved Document B expects each habitable room on the upper floor to have either an emergency escape window or direct access to a protected stair. In practice, that means a new bedroom over the extension needs an escape window: an openable area of at least 0.33 square metres, at least 450mm high and 450mm wide, with the bottom of the opening no more than 1100mm above the floor. Our page on fire escape window regulations covers the details, including restrictors and the ground below.
Adding habitable rooms also brings in smoke alarms: mains-powered alarms with a standby supply, interlinked, in the circulation space on each storey. See smoke alarm regulations. Where the extension wall is within 1 metre of the boundary, it must be fire-resisting with only very small unprotected areas such as windows, which often decides where you can put a side window.
Insulation and glazing
Approved Document L, Volume 1 sets the same limiting U-values as for any extension: 0.18 W/m²K for walls and floors, 0.15 for the roof, 1.4 for windows and doors that are more than 60% glazed, and 2.2 for rooflights. The total area of windows, glazed doors and rooflights should not exceed 25% of the extension's floor area, plus the area of any existing windows or doors it covers. On a two storey extension the floor area normally counted is the total across both storeys, which allows more glass than the same footprint on one floor; confirm the figure with your building control body early. Going over is possible, but your designer must then show by calculation that the whole house performs no worse than a compliant design. These are the figures in the 2021 edition of Approved Document L, which is current; a 2026 edition takes effect in England on 24 March 2027, with the same limiting U-values for work to existing homes, so check which edition applies if your application goes in after that date.
Bathrooms, ventilation and drainage upstairs
Many two storey extensions add an en suite or family bathroom on the first floor. That brings in:
- Drainage: a new soil and vent pipe, or a connection to the existing stack, with fall and access for rodding. Building over or within 3 metres of a public sewer needs the water company's agreement.
- Extraction: an intermittent fan of at least 15 litres per second in a bathroom, and more in a kitchen. See extractor fan regulations.
- Electrics: bathroom zones and RCD protection under Part P. Our en suite regulations page goes through the layout rules.
Every new habitable room also needs purge ventilation, usually an openable window, and background ventilation such as trickle vents.
Stairs and the existing house
If the extension is reached through the existing first-floor landing, the stair is usually unchanged. If you alter or move it, the new stair must meet Part K: a maximum pitch of 42 degrees for a private stair, rises of 150 to 220mm and goings of 220 to 300mm, and at least 2m headroom. See our staircase regulations page. Check the boiler and hot water too: more bathrooms and radiators may need a larger system.
How approval works
For a project of this size, use a full plans application to your council's building control or a registered building control approver. The plans are checked before work starts, a decision is due within five weeks (or two months if you agree), and inspectors then visit at key stages: foundations, damp-proof course, drains, structure and completion. A building notice, where you start with no plan approval, is legally possible on many projects but risky here, because problems surface after they are built.
Since October 2023 the dutyholder rules from the Building Safety Act also apply to home extensions: the client, designer and builder each have duties to make sure the work complies. GOV.UK's Building Safety Act guidance explains the regime. At the end you get a completion certificate, which you will need when you sell. If work was done without approval, see our regularisation certificate page.
Planning: the 3m and 7m rules
On a house in England (not a flat or maisonette), a two storey rear extension can be permitted development under Class A if:
- it extends no more than 3m beyond the rear wall of the original house;
- it is at least 7m from the rear boundary;
- its roof pitch matches the existing house as far as practicable, and it is no higher than the existing roof;
- upper-floor windows in the side walls are obscure glazed and fixed shut below 1.7m above the floor;
- materials are similar in appearance to the house, and there is no balcony or raised veranda.
Two storey side extensions are never permitted development, and in conservation areas any extension of more than one storey needs planning permission. Many London houses have had permitted development rights removed too, so check the property's planning history before relying on them. The Planning Portal has the full conditions, and our conservation area guide explains the extra controls.
If you dig within 3 metres of a neighbour's building and below their foundations, or build on the boundary line, the Party Wall etc. Act 1996 applies; see our party wall guide.
Designing the rooms at the same time
The structure and the regulations shape the interior more on a two storey extension than almost any other project: the steel positions set ceiling lines on both floors, the escape window fixes the bedroom window size, and the soil stack decides where the bathroom can go. Settle the layouts before the drawings go to building control. For the single storey version, see single storey extension building regulations, or return to the Vertigo Interiors homepage.
Frequently Asked Questions
Do you need building regulations for a two storey extension?
Yes. A two storey extension always needs building regulations approval, covering structure, fire safety, insulation, ventilation, drainage and electrics, whether or not it also needs planning permission.
Can you build a two storey extension without planning permission?
Sometimes. On a house in England, a two storey rear extension can be permitted development if it projects no more than 3m beyond the original rear wall and stays at least 7m from the rear boundary, among other conditions. Two storey side extensions, flats and houses in conservation areas need planning permission.
What is the 7 metre rule for two storey extensions?
Under permitted development, any part of an extension with more than one storey must be at least 7 metres from the boundary of the land opposite the rear wall of the house. If it would be closer, you need planning permission.
Do first floor bedrooms in an extension need escape windows?
In a typical two storey house, Approved Document B expects each new upper-floor habitable room to have an emergency escape window, unless it opens directly onto a protected stair. An escape window needs an openable area of at least 0.33 square metres, at least 450mm high and wide, with the bottom no more than 1100mm above the floor.
Do you need a structural engineer for a two storey extension?
In practice, yes. Building control will want calculations for the foundations, the steel beams where the extension opens into the house, the first floor joists and the roof, and an engineer normally prepares them.
How long does building control approval take for a two storey extension?
A full plans application must be decided within five weeks, or two months if you agree an extension. Inspections then happen at set stages during the build, and you receive a completion certificate at the end.