Disproportionate collapse and building class in design
The consequence class a building falls into is fixed by storey count, use and floor area, all of which are settled at planning stage. What each class requires and where the thresholds bite.
Disproportionate collapse is the requirement that stops a building coming down further than the event that started it. Requirement A3 of the Building Regulations puts it plainly: the building shall be constructed so that in the event of an accident the structure will not suffer collapse to an extent disproportionate to the cause. Approved Document A turns that into a classification, and the disproportionate collapse building class a scheme falls into decides the tying, the frame and part of the cost.
Architects meet it late, usually when the engineer explains why a fifth storey changed the structural strategy. It is worth meeting earlier, because the class is fixed by height, use and floor area, which are planning decisions.
How buildings are classified
Approved Document A sets out consequence classes derived from the Eurocode approach. The boundaries are set by building use, storey count and floor area per storey.
| Class | Typical scope |
|---|---|
| 1 | Houses of up to four storeys, agricultural buildings, buildings people rarely enter |
| 2A | Flats, offices and hotels of up to four storeys, five storey single occupancy houses, retail of up to three storeys under 1,000 square metres per storey, single storey educational buildings |
| 2B | Flats, offices, hotels, educational and retail buildings above the 2A limits and up to 15 storeys, hospitals up to three storeys, car parks up to six storeys |
| 3 | Anything exceeding the 2A and 2B limits on storeys or area, large grandstands, buildings holding hazardous substances or processes |
Buildings to which the public are admitted are treated by area as well as height, with thresholds that move a building from 2A into 2B and then into Class 3 as the floor area per storey rises.
Mixed use buildings take the highest class that applies to any part, which catches residential over retail more often than designers expect.
What each class has to provide
The provisions escalate.
Class 1 needs nothing beyond satisfying the other structural requirements. Ordinary stability and the normal detailing rules do the work.
Class 2A requires effective horizontal ties, or effective anchorage of suspended floors to walls. On a masonry building that means strap anchors and continuity through the floor plates rather than a separate exercise.
Class 2B is where the design changes. Horizontal ties are required along with vertical ties in every supporting column and wall. The alternative is a notional removal check: take away each supporting column, each beam supporting a column, and each nominal length of load bearing wall in turn, and show the building remains stable with the area at risk of collapse limited to 15 per cent of the floor area of that storey or 100 square metres, whichever is smaller, in each of two adjacent storeys. Where an element cannot be removed within those limits, it is designed as a key element capable of resisting an accidental design load of 34 kilonewtons per square metre applied from any direction.
Class 3 requires a systematic risk assessment covering foreseeable and unforeseeable hazards, which is a specialist exercise rather than a code check.
Storey counting and the traps
The class turns on a number, so the counting rules matter.
Basement storeys need not be counted provided they meet the requirements for Class 2A. That single note has saved a lot of schemes, and missing it has pushed others into an unnecessary class.
Three other counting questions come up repeatedly. Whether a roof level plant enclosure or a stair overrun counts as a storey. Whether a mezzanine counts. And how to treat a building stepping down a slope, where the storey count differs across the plan. None of these has a universal answer in the Approved Document, so the engineer records the interpretation and the reasoning, and that record goes to building control rather than being settled verbally.
The threshold that gets confused with all of this is the higher-risk building definition under the Building Safety Act framework, which turns on 18 metres or seven storeys with at least two residential units. Consequence class and higher-risk status are different tests with different consequences, and a building can be 2B without being higher-risk.
Effect on frame choice and layout
Tying requirements are easier to satisfy in some structural forms than others.
In situ reinforced concrete with continuous reinforcement through the slab provides tying almost inherently, which is why flat slab residential buildings rarely struggle at Class 2B. Steel frames with simple connections need the ties checked explicitly, and the connection design often governs. Precast concrete and load bearing masonry demand the most detailing attention, because continuity has to be created rather than assumed.
Layout matters too. Long span transfer structures, columns sitting on beams over an open ground floor, and single columns supporting large tributary areas all create elements that fail the notional removal check and attract that designation by default. An undercroft car park or a ground floor retail unit under flats is the classic case, and the earlier the engineer sees the arrangement the cheaper the answer.
The same interaction between structural arrangement and design freedom appears in facade retention schemes, where the temporary and permanent conditions both have to be resolved.
Where planning decisions fix the answer
Storey count, use and floor area per storey are all determined by the planning application. By the time permission is granted, the consequence class is effectively settled.
Two decisions deserve a structural sanity check before submission. Adding a storey to take a residential block from four to five moves it from Class 2A to Class 2B, with vertical tying or removal checks and a different cost base. And increasing floor plate area on a building open to the public can cross an area threshold without any change in height at all.
Where a scheme reaches 18 metres or seven storeys with two or more dwellings, a Fire Statement is required with the planning application under the Gateway One arrangements, and that is a document requiring fire engineering competence rather than a drafting exercise. A licensed or competent professional is legally required for it, and an AI-assisted draft is a starting point for that professional rather than a substitute.
Information to carry forward
Record three things at the end of the planning stage and keep them with the design.
- The consequence class, with the reasoning for the storey count and the area figures it rests on.
- The chosen compliance route, whether full tying or notional removal, and any elements identified as requiring special treatment.
- The design assumptions the architect must not change without asking, such as column positions, transfer locations and the extent of load bearing walls.
That third item is the one that saves money. A layout change that moves a column off a grid after the structural strategy is fixed can turn an ordinary frame into one carrying elements designed for accidental loading. Where an existing building is being altered or extended, a Structural Survey establishing what is actually there is the necessary starting point, and councils expect a suitably qualified author for it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice, so check the current Approved Document A and take advice from a chartered structural engineer before relying on it. Enter your postcode on PlanScope to see which reports a taller residential scheme is likely to need.
Frequently asked questions
What are the building consequence classes?
Approved Document A sets Class 1 for houses up to four storeys and similar low risk buildings, Class 2A for flats, offices and hotels up to four storeys, Class 2B for those above the 2A limits up to 15 storeys, and Class 3 for anything exceeding the 2A and 2B limits on storeys or floor area per storey.
What does Class 2B require?
Horizontal ties plus vertical ties in all supporting columns and walls, or a notional removal check. That check removes each supporting element in turn and shows the collapse area stays within 15 per cent of the storey floor area or 100 square metres, whichever is smaller, in each of two adjacent storeys.
Do basement storeys count towards the class?
Not necessarily. Approved Document A allows basement storeys to be left out of the count provided they meet the requirements applying to Class 2A. Missing that note pushes schemes into a higher class than they need to be, and relying on it without recording the reasoning causes trouble at building control.
Is consequence class the same as a higher-risk building?
No. Higher-risk status under the Building Safety Act framework turns on 18 metres or seven storeys with at least two residential units, and triggers the Gateway process. Consequence class is a structural classification in Approved Document A. A building can be Class 2B without being higher-risk, and the two are often confused.
What happens when an element cannot be notionally removed?
It is designated under Approved Document A and designed instead to resist an accidental design load of 34 kilonewtons per square metre applied from any direction. Transfer structures and single columns carrying large tributary areas frequently fall into this category, which is why an undercroft car park below flats needs the engineer involved early.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.