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Building regulations for open plan/duplex flats in residential buildings (exceeding 4.5m in height)

This note gives guidance on considerations to be made when assessing compliance with Requirement B1. The guidance specifically relates to situations where the proposals do not meet the narrative guidance given in Approved Document B or BS 9991 in regard to the layout of open plan flats or duplex flats in residential buildings with height exceeding 4.5m.


When the layout of an open plan flat or duplex flat does not follow the narrative guidance given in AD B or BS 9991, the compliance of the layout is reliant on an engineered approach. A check list is appended of the design guidance that must be met so that an engineered justification is not needed.

Currently there is no definitive approach and this can lead to difficulties when the checking engineer disagrees with principles employed by the design engineer. With the lack of definitive guidance these disagreements can be subjective in nature and outside of the Building Control Surveyor’s competence to arbitrate.

For this reason, this note lays down the principles which our checking engineers have agreed the design should follow. These should be passed to the design team as early as possible, when applicable, so they do not carry out abortive engineering (it would be acceptable to send this note by way of explanation).

The Guidance in BS 9991:2015 restricts open plan kitchens to 8m x 4m; we are able to consider proposed deviations to this guidance from a competent fire engineer. These should be based on the conclusions in the University of Manchester research paper entitled “Fire safety design of open plan apartments in England”, for flats with open plan kitchens up to a size 16m x 12m that otherwise follow the guidance in BS 9991:2015.

The principles laid out below were agreed through a series of meetings with our checking fire engineers competent to consider this form of design.


The approach to design we would accept for checking would compare the following in the proposed design and the similar design permitted by guidance.

  • Time of smoke detection (to give an indication of the time that occupants are first alerted). After this it is reasonable to assume that the range of pre-movement times outlined in BS 7974-6 would be the same for both flats.
  • Time of sprinkler actuation and cap the fire size at that moment. If an ultra fast growth fire in a pan is used to do this, then it may be worth carrying out a sensitivity study to see if there is a vast difference if a fire is medium growth. For example, the kitchen is considered a high-risk area and a fire could originate in a toaster /hob etc and spread to surrounding cupboards/contents etc (without an oil pan involved).
  • Compare conditions at different time points in the analysis to see which design has the better conditions.
  • Use Fractional Effective Dose devices to ascertain at what time occupants are being exposed to a high level of contaminants in any particular area.
  • Consider any further distances that occupants may need to travel in each of the designs.

The following rationale was discussed in arriving at the above outline. If the analysis is deterministic, the purpose is to demonstrate that safe conditions can be achieved if the kitchen is not enclosed (as is required by guidance for open plan flats of this size). The analysis therefore should consider a fire in the kitchen, not a fire confined to the pan only. Tenability across a range of pre movement times is rarely achievable for anything larger than a pan fire. At the time of discussion, it was estimated that 90% of CFD studies submitted are comparative studies. Any fire larger than 175kW is certainly possible and would likely result in a fail for a deterministic study.

The designs from the NHBC reports and BS 9991 provide approaches that have been considered acceptable based on risk (from the CRISP simulations carried out). They do not guarantee tenability is achieved in all cases, only that the risk is considered acceptable.

If the kitchen is not being enclosed, this is a departure from guidance that should be justified by showing that the proposals are no worse than the designs considered acceptable. This could be because the apartment has a larger volume, higher ceilings, increased smoke detection, a particular layout, increased suppression, hob suppression etc. This may need to be offset against slower detection and suppression activation or longer travel distances etc. That is where the CFD modelling comes into play.

It is acknowledged that pan fires are often used to establish heat flux and separation distances. In our view, they are not the only fire that should be considered when the proposal to not enclose the kitchen is being assessed.

It was concluded that a comparison is currently the preferred approach with these types of analysis. Our checking engineers are unlikely to accept a deterministic analysis that only considers a pan fire of 175kW.

Consideration of detection in open plan kitchens

As open plan flats generally have the kitchen and living area combined, in some cases located on the escape route, the location and type of detection in these areas needs to be carefully considered.

BS 5839-6:2019 recommends that smoke detectors should not be used in any room where the smoke detector would have a high potential of false alarm (such as a kitchen) unless the use of other forms of detection (such as a heat detector) is precluded on the basis of their speed of response.

A heat detector is normally specified due to concerns relating to false alarms. However, heat detectors respond much more slowly than a smoke detector and are unlikely to operate early enough to provide sufficient early warning to sleeping occupants. In cases where the open plan area forms part of the means on escape/circulation route heat detectors are not suitable as the only form of detection.

Please download the PDF below to view our open plan flat checklist.

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The Guidance Note does not constitute professional advice and Sweco hereby offers no reliance and accepts no liability (in contract, tort (including negligence) or otherwise) for parties acting on the content of this Guidance Note without taking specific professional advice from Sweco. The Guidance Note is provided as generic guidance only and any opinion, views or advice expressed in this Guidance Note are made without any representations or provide any assurances or warranties as to the content.  If you require specific advice with respect to your own project or circumstances, please contact Neil Badley.