A loft conversion in a house needs a protected escape route under Approved Document B, Volume 1 once the new storey sits more than 4.5m above ground level. In practice that means a stair enclosed in fire-resisting construction of at least 30 minutes (REI 30), doors rated at least E 20, and mains-powered smoke alarms.
Part B is the building regulations requirement for fire safety, and for a loft it is usually the part that shapes the floor plan. The protected route costs floor area, so it is worth settling on paper before anyone orders a dormer. Planning permission or permitted development does not change any of it.
Key Takeaways
- The trigger is height: a new storey more than 4.5m above ground level needs its full escape route addressed (paragraph 2.21 of Approved Document B, Volume 1).
- The protected stairway is separated by REI 30 construction and runs to a final exit. Doors on it need at least E 20, and existing doors are upgraded where necessary.
- Smoke alarms must be mains operated, to BS EN 14604, with a standby power supply, at Grade D2 Category LD3 as a minimum.
- Escape windows are specified for storeys up to 4.5m. The loft room itself relies on the protected stair, not a window.
- Sprinklers are an alternative route in specific layouts, not a default.
When does Part B apply to a loft conversion?
Part B applies to a loft conversion because requirement B1 covers building work to existing buildings as well as new ones. The specific trigger in paragraph 2.21 is a new storey created above 4.5m. The height is measured to the floor of the top storey, as shown in Diagram D6 of Appendix D to the document.
A loft over a two-storey house will often sit above that line, so measure from your survey rather than assume. Separately, paragraph 1.8 requires a fire detection and alarm system wherever a new habitable room is provided above or below the ground storey, and a loft bedroom is exactly that. The document is the 2019 edition incorporating the 2020, 2022, 2025 and 2026 amendments, collated with the 2029 changes.
What is a protected escape route?
A protected stairway is a stair separated from the rest of the house by fire-resisting construction of at least REI 30 at every storey, leading either to a final exit or to two ground-level exits separated from each other. Paragraph 2.5(a) sets this out. Above it, cavity barriers or an EI 30 ceiling should be provided in the roof void.
For a loft, paragraph 2.21 adds two duties: the full extent of the escape route from loft to outside must be addressed, and fire-resisting doors and partitions provided, including upgrades to existing doors. Where the ground floor is open plan, new partitions are needed to enclose the stair. That is the usual cost of a loft conversion in floor area, and the reason layouts are tested early.
| Element | What Approved Document B Volume 1 asks for | Paragraph |
|---|---|---|
| Escape stair | Protected stairway, minimum REI 30, to a final exit | 2.5(a) |
| Doors on the route | Fire doorsets, minimum E 20 | 2.21(b) |
| Partitions on the route | Minimum REI 30 | 2.21(b) |
| New floors | Minimum REI 30 | 5.4 |
| Smoke alarms | Mains operated, standby supply, minimum Grade D2 Category LD3 | 1.1 to 1.4 |
| Escape window, where required | 0.33m² openable, at least 450mm by 450mm, bottom at most 1100mm above floor | 2.10 |
Which fire doors does a loft conversion need?
Doors opening onto the protected route need a fire doorset rated at least E 20, meaning 20 minutes of integrity, and existing doors must be upgraded where necessary. Within a dwellinghouse those doors do not have to be self-closing. Paragraph C5 of Appendix C exempts fire doorsets within dwellinghouses, and C12 removes the fire door signage requirement for them.
Doors between a house and an integral garage are different: they must be self-closing. The 2029 amendment leaves both points as they are. Where an old door has historical or architectural merit, paragraph 2.22 says the possibility of retaining and upgrading it should be investigated rather than replacing it. Appendix C expects test or assessment evidence to cover the complete installed assembly, down to the hinge materials.
What smoke alarms does a loft conversion need?
Alarms must be mains operated, conform to BS EN 14604 and have a standby power supply such as a battery or capacitor. The minimum system is Grade D2 Category LD3 under BS 5839-6 (paragraphs 1.1 to 1.4). A new loft room also needs the circulation spaces of the house covered by alarms under paragraph 1.9.
Larger houses step up. A large dwellinghouse has more than one storey and at least one storey over 200m². With two storeys it needs Grade A Category LD3; with three or more, Grade A Category LD2 (paragraphs 1.5 to 1.7). A loft can tip a big house from two storeys to three.
One caution. The document does not use the word interlinked, and the installation detail sits in BS 5839-6, which is a paid standard we have not reproduced. Ask your building control body how they expect the alarms to be wired before the electrician starts.
Do loft conversions need escape windows?
The loft room itself does not need an escape window under Approved Document B, because for storeys above 4.5m the route is the protected stair. Escape windows are specified for habitable rooms on storeys up to 4.5m, and they matter in the alternative layout in paragraph 2.23, which relies on a first storey escape window.
Where a window is required for escape, paragraph 2.10 sets the size: an unobstructed openable area of at least 0.33m², at least 450mm high and 450mm wide, with the bottom of the opening no more than 1100mm above the floor. Locks and child-resistant stays are allowed, and the window must stay open without being held.
What are the alternatives if a protected stair will not fit?
Approved Document B gives two. Paragraph 2.5(b) lets a top storey be separated from the floors below by REI 30 construction and given its own alternative escape route to a final exit. Paragraph 2.23 offers a sprinkler-based alternative for open-plan layouts.
The paragraph 2.23 route has three parts, all of which apply: sprinkler protection to the open-plan areas, an REI 30 partition and E 20 door separating the ground storey from the upper storeys (with access to a first storey escape window), and cooking facilities separated from the open-plan area by REI 30 construction. Sprinkler systems in residential buildings follow BS 9251 (Appendix E, paragraph E3). Sprinklers also appear in paragraph 2.6, for houses with two or more storeys above 4.5m: on top of the protected stairway, either an alternative escape route from each storey above 7.5m or a sprinkler system throughout. We give no sprinkler cost, so get a quotation before choosing this route.
What fire resistance do the new floor and walls need?
Where you add a storey to a two-storey single family dwellinghouse, new floors need a minimum of REI 30 and so does any floor forming part of the enclosure to the route to the final exit. The existing first-storey construction should be at least R 30. That is paragraph 5.4, and it is the clause that usually decides how the floor is built.
Integrity and insulation may be reduced where both conditions hold: only one storey is added, with at most two habitable rooms, and the new storey is no more than 50m² in total. Your structural engineer will design the floor around this, which is why Part A and Part B drawings should be coordinated. See our guide to Part A and when calculations are needed.
How does Part B show up on your drawings and fee?
Building control reads the protected route from the drawings: the stair enclosure, door positions with ratings, partitions, alarm positions and any sprinkler layout. Building regulations drawings from Fixed Fee Plans are £795, fixed, and show these on the sheet. Your council or a registered building control approver sets the building control charge separately.
Beams and floor structure need an engineer's calculations, from £495 as a separate service. Our loft conversions page covers the process, the building regulations drawings page lists stages and prices, and the structural calculations service covers the engineering. The insulation side of a loft is in our guide to Part L for extensions.
Frequently asked questions
Do I need sprinklers for a loft conversion?
Not by default. They appear in paragraph 2.23 as an alternative for open-plan layouts, and in paragraph 2.6 as one of two options for houses with two or more storeys above 4.5m.
What if my loft floor is 4.5m or lower?
Paragraph 2.2 then applies. Each habitable room needs an escape window or external door, or direct access to a protected stairway.
Have the recent amendments changed the rules for house lofts?
The document says the 2026 amendments concern a second stair and evacuation lifts in blocks of flats, and the 2029 amendments concern the removal of national classes for fire resistance. The paragraphs cited here are as printed in the current collated edition.
Sources
All retrieved 10 October 2026.
- Approved Document B, Volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments collated with 2029 amendments (paragraphs 1.1 to 1.9, 2.2, 2.5, 2.6, 2.10, 2.19 to 2.23, 5.4; Appendices C, D and E): https://assets.publishing.service.gov.uk/media/6aa3bb335f6e942efe37f150/AD_B1_2026.pdf
- Fire safety: Approved Document B publication page: https://www.gov.uk/government/publications/fire-safety-approved-document-b