Does a two-storey extension need a protected stairway?

Yes. Any dwelling with two or more storeys requires a protected stairway — the walls, floor and ceiling enclosing the stairway must provide 30 minutes fire resistance. All doors opening onto the stairway must be FD30S (30-minute fire doors with self-closers). This applies to new two-storey extensions and loft conversions.

What is an FD30 fire door?

An FD30 fire door provides 30 minutes fire resistance. For domestic stairways, FD30S doors are required — the 'S' denotes a smoke seal. Each door must have intumescent strips (which expand in heat to seal the gap) and cold smoke seals. Doors must be self-closing. Third-party certified doors with a label/plug are recommended by Building Control.

What smoke alarms do extensions need?

All extensions creating new habitable rooms need mains-wired interlinked smoke alarms. A minimum of one per floor level on the escape route (hallway/landing). Kitchen requires a heat alarm (not smoke, to avoid false alarms). Alarms must be interlinked so when one triggers, all sound. Battery-only alarms are not acceptable for new work.

Do first floor bedrooms need escape windows?

Yes. First floor (and second floor in loft conversions) habitable rooms need either: access to a protected stairway AND an escape window, or access to a protected stairway that leads directly to a final exit. Escape windows must have an openable area of at least 0.33m² with no dimension less than 450mm.

What fire safety is needed for a loft conversion?

A loft conversion creating a third storey has the most stringent requirements: protected stairway from ground floor to loft, FD30S doors on ALL floors (existing doors may need upgrading), mains-wired interlinked smoke alarms on every level, escape window(s) in loft bedrooms, and 30-minute fire resistance to all stairway enclosure elements.

TradeCalculator

Extension Fire Safety Calculator — Part B Compliance

Determine fire safety requirements for two-storey extensions and loft conversions: protected stairway, FD30 doors, mains-linked smoke alarms and escape windows.

Extension type

Existing house storeys

New first floor bedrooms

Open plan kitchen/living

Inner room (escape through another)

How We Calculate This

This calculator determines Part B fire safety requirements based on the number of storeys and extension configuration.

Key requirements

Protected stairway: required for 2+ storey dwellings
FD30S doors: on all rooms opening to the protected stairway
Smoke alarms: mains-wired interlinked, minimum one per floor
Escape windows: 0.33m² openable area, max 1100mm sill height

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.