Protected lobbies in a large house converted to flats

In a large house being converted to flats, the common stair decides how many units the building can take, and protected lobbies are usually the price of keeping a single stair. Approved Document B expects dwellings to be separated from a common stair that is the only escape route, by a protected lobby or a common corridor, and a small single stair building has further limits on height and on the number of flats per storey. Each lobby costs floor area on every level, which is why a five unit scheme on paper often becomes four once the fire strategy is drawn.

Get a fire engineer onto the layout at sketch stage. The alternative is redesigning after building control comments, with planning permission already granted for a unit mix you can no longer build.

Why a common stair drives the whole layout

An existing house stair was designed to serve one household. Converting the building creates a common escape route serving separate dwellings, and the standards that apply are those for flats rather than those for a house.

Two consequences follow immediately. The stair enclosure has to be fire resisting, which means the walls, the doors onto it and often the floor construction below it. And whatever opens onto that stair has to be controlled, because smoke entering the only escape route from a flat on fire puts everybody above at risk.

Approved Document B sets out provisions for small single stair buildings, including a limit on the height of the top storey above ground level, commonly expressed as 11 metres, and a limit on the number of dwellings per storey. Read the current version of the Approved Document for the building you have, because the provisions have been amended more than once and the detail turns on the building's actual height and arrangement.

BS 9991 offers an alternative route for conversions, with more flexibility where a fire engineer takes responsibility for the design. That flexibility is useful when the house will not accept a lobby without losing a bedroom.

Protected lobbies and where they are needed

A protected lobby is a fire resisting enclosure between the flat entrance door and the common stair, with two doors in series so that smoke from a flat has to pass through a second barrier before it reaches the escape route.

The arrangement is needed where the stair is the only escape route and dwellings would otherwise open directly onto it. A common corridor serving two or more flats on a storey performs the same function along a horizontal route, and the choice between a lobby and a corridor usually comes down to how the existing plan divides.

Three practical points decide whether a lobby fits.

The existing landing is often too small. A lobby needs enough depth that the two doors can both open without colliding, and enough width for the door to swing in the direction of escape.

The lobby has to be free of risk. A meter cupboard, a boiler or a store opening into a lobby is normally unacceptable unless separately enclosed, and services passing through need fire stopping and, where appropriate, intumescent collars.

Ventilation matters. Smoke control in a lobby or corridor is usually provided by an openable vent or an automatic opening vent at the head of the stair, and the arrangement forms part of the engineered design rather than a detail to be added later.

Fire door specification and self closing devices

Flat entrance doors onto a protected lobby or common corridor are normally specified as 30 minute fire resisting with smoke seals, fitted with a self-closing device. The smoke seal is the part most often omitted on site and most often picked up on inspection.

Specify the whole doorset rather than the leaf. A tested doorset includes the leaf, frame, intumescent and smoke seals, hinges, latch and closer, and the certification covers that assembly. A new leaf hung in an old frame on old hinges is not a fire door, whatever the leaf is stamped with.

Where an existing panelled door of historic interest is to be retained, upgrading is sometimes possible with an assessed solution, and that needs evidence from a competent person rather than a product leaflet. In a listed building this conversation happens with the conservation officer as well as building control.

Self-closing devices on flat entrance doors are a management item as well as a construction item. Overhead closers get disconnected by residents, and the duty to check them in the common parts of a building with multiple dwellings now sits squarely with the responsible person under fire safety legislation.

Alarm and detection strategy for a conversion

Detection in a conversion runs at two levels, and they have to work together.

Within each flat, detection to the dwellings code of practice is normally provided, mains powered with battery backup, covering the escape route within the flat and the main habitable rooms depending on the grade and category selected.

In the common parts, a detection system may be required to operate ventilation, or to give warning where the escape route arrangement depends on it. Where a stair relies on an automatic opening vent, detection triggers it.

The point to settle early is whether any signal passes between the two. A system that sounds an alarm in every flat when one flat's detector operates changes evacuation strategy entirely, and it is rarely what a small conversion needs. Set out the strategy in writing, with the grade and category for each part, and have it designed by a competent person. A Fire Safety Strategy to BS 9991 is a fire engineering document, and a competent fire engineer is legally required to take responsibility for it. An AI-assisted draft is a starting point for that professional rather than a substitute.

Effect on unit layouts and lost area

Here is the arithmetic that catches investors. A lobby takes two to four square metres of circulation on each floor it serves, and that area comes out of the adjoining flat.

On a typical Victorian or Edwardian house of three storeys plus a basement, the losses compound. The ground floor flat loses its hall. The first floor flat loses the front corner of what was the principal bedroom. The second floor unit, already the smallest, drops below the minimum gross internal area for a one bedroom flat and has to merge with something.

Four responses are available, in rough order of cost.

1. Rearrange the flats so that the unit adjoining the stair on each floor is the one that can afford the area.
2. Reduce the unit count and increase the size of the remaining flats, which often improves the rent per square metre anyway.
3. Take an engineered approach under BS 9991, which may allow a different arrangement supported by suppression or by enhanced detection.
4. Form a second escape route, which on a terraced house usually means an external stair at the rear and a separate planning application.

Basements deserve their own thought, because escape from a basement flat interacts with flood risk and with light wells, and the two are best designed together as basement light wells, escape and flood risk describes. Loft conversions raise the mirror image problem, where the structure rather than the escape route sets the limit, and structural limits on a terraced house loft conversion covers that ground.

Reports and drawings to submit

Planning and building control want different things, and conflating them causes delay.

For the planning application on a house to flats conversion, the documents that matter are the floor plans with every room dimensioned and areas scheduled, the elevations showing any new openings, rooflights or vents, a parking and cycle store layout, a refuse store, and a short statement on the standard of accommodation. Fire escape is generally not a planning matter at this scale, since the fire statement requirement at Planning Gateway One applies to relevant buildings of 18 metres or more, or seven or more storeys, with two or more dwellings.

For building control, submit a full plans application with a fire strategy drawing that shows:

• The protected stair enclosure and the fire resistance of each element.
• Every lobby and corridor, dimensioned, with door swings.
• A door schedule giving the doorset reference, rating and seals for each opening.
• Detection positions, grade and category for the flats and for the common parts.
• Smoke ventilation arrangements and what triggers them.
• Travel distances from the furthest point in each flat.

Where load paths change to form the lobby walls, a Structural Survey of the existing fabric informs the design and reduces the chance of an expensive discovery during the strip-out.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and have the fire strategy prepared by a competent fire engineer.

For specialist technical reports, qualified professional input may be required. Enter the building's postcode on PlanScope to see which documents a conversion is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.