By Grayling Thomas Architects · October 2026

BB104 (Building Bulletin 104) is the Department for Education's area guidance for special schools, alternative provision (AP), specially resourced provision (SRP) and Units in England. It sets out "non-statutory area guidelines for buildings (Part A)" and guidance on grounds (Part B) for ages 3 to 19. For SEN school design it gives recommended floor areas as ranges rather than fixed numbers: classroom sizes matched to pupil need, totals for each category of space, circulation allowances, toilet and hygiene room provision, and site areas. A primary classroom for ambulant pupils, for example, has a recommended standard size of 55m2 for a group of 12.

What is BB104 and who is it for?

BB104 is published by the Department for Education on the Area guidelines for SEND and alternative provision (BB 104) page on gov.uk. That page shows it as published on 11 February 2026. The PDF itself is headed Building bulletin 104, December 2015. "The area guidelines supersede the recommended areas in Building Bulletin (BB) 102."

The document says it aims to help "those involved in briefing for and designing new buildings, refurbishment or conversion projects, including sponsors and architects". It is also written for headteachers and governors who need advice on how much space to provide.

The single most important sentence for anyone reading it is this one: the guidelines "will not necessarily have to be met in every case and should always be applied flexibly in light of particular circumstances." BB104 is a briefing tool. It is not a building regulation, and it does not replace Approved Document M on access, which it refers to throughout.

BB104 covers four settings:

• Special schools. All pupils have education, health and care (EHC) plans or statements of SEN. The guidance notes that "most schools have around 100 pupils".

• Alternative provision (AP). For pupils who cannot attend mainstream school, split in BB104 into AP (health) and AP (behaviour).

• Specially resourced provision (SRP). Extra facilities on a mainstream site, where pupils spend "usually well over 50% of their timetable" in mainstream classes.

• Units. Also on a mainstream site, for "a small number of pupils, typically less than 30", who spend most of their time in the Unit.

Why do SEND settings need more space than mainstream schools?

BB104 is explicit that every one of these settings needs more area per pupil place than a mainstream school. The reasons are practical. Pupils are "taught in smaller groups, averaging around 8 to 12 and as low as 4 to 6 where pupils need extensive support". Staff ratios are higher, particularly in special schools "where 2 or 3 teaching assistants or support staff work alongside the teacher or give support in a separate space".

Equipment drives space too. "One child with a wheelchair and/or mobility aids may need as much space as three ambulant pupils." Some pupils "may use up to three different pieces of mobility equipment during the school day", and all of it has to be stored somewhere when it is not in use.

How do the BB104 formulae work?

Every BB104 formula uses a base area plus an area per pupil place. The pupil number is normally the planned capacity: "the number of places planned rather than the number on roll". The graphs then show a zone for each category of space. The top of the zone is the recommended maximum and the bottom is the recommended minimum.

For a primary special school for ambulant pupils, for instance, the recommended net area runs from "minimum 475 + 10N" to "maximum 575 + 10.7N", where N is the number of pupil places.

Net area, the usable space, is split into five categories:

1. basic teaching

2. halls, PE, dining and social

3. learning resource areas

4. staff and administration

5. storage

Add the minimum for each category together and the total falls short of the recommended minimum net area. BB104 calls the difference the float, which a school can spend on its own priorities, "for example providing larger classrooms or an additional therapy room."

Non-net area covers toilets and personal care, commercial kitchens, circulation, plant and internal walls. Net plus non-net gives the gross internal floor area. For new buildings BB104 expects:

In existing buildings the ratio can rise to "as much as 175%" in a special school or AP.

BB104's advice on using the ranges is worth quoting in full: "it is important to make use of the whole area range in this guidance and not to assume the minimum (or maximum) will automatically apply."

What size should an SEN classroom be under BB104?

This is the question most briefs start with. Figures 20 and 21 of BB104 give a recommended range and a recommended standard size for the most common teaching spaces. A selection:

Two points stand out. A classroom for six non-ambulant pupils is larger than one for twelve ambulant pupils, because wheelchairs, standing frames and horizontal learning stations need room to move. And the group sizes are indicative; BB104 says "settings will vary."

Below the standard size, a room starts to lose flexibility. BB104 gives examples: no space for a sensory corner for pupils with PMLD or a soft seating corner for pupils with MLD, fewer cookers in a food room, no room for a drum kit or piano. Below the minimum is a different matter: "An area below the recommended minimum area is not recommended and could pose a health and safety risk, particularly in a practical room."

BB104 also warns against designing rooms too tightly around today's pupils. In special schools for MLD/SLD, "rooms large enough for groups of at least twelve are generally recommended" even though the average group size may be around ten.

Other rules of thumb from the basic teaching section:

• Primary classrooms should have water and drainage for light practical work, unless the room is at the bottom of its range and practical work happens elsewhere.

• Units should include classrooms for "(typically) 6 to 8 pupils".

• SRP has no basic teaching area, because pupils have their lessons in the main school.

Circulation, corridors and storeys

Circulation is where SEND buildings differ most visibly from mainstream ones. BB104 sets minimum circulation as a share of total net area:

• 22.5% of the total net area for AP, SRP and Units (except settings for pupils with PD);

• 25% for SRP and Units for pupils with PD;

• 27% of the total net area for ambulant special schools;

• 32% of the total net area for non-ambulant special schools.

Where pupils are mainly ambulant, corridors serving more than two classrooms need "a minimum clear width of 1.8m", or 2m where pupils need more personal space. Where pupils use mobility aids or need support workers, the figure rises to a "minimum clear width of 2.2m".

"Special schools should ideally be single storey". Where an upper floor cannot be avoided, stairs must let staff assist a pupil from both sides and take an evacuation chair, and lifts must carry a child, their equipment and staff together.

Plant needs "at least 2% of the total net area" in special schools and AP. Internal walls take "at least 3% of the net area, and up to 6%" where walls are thicker.

Toilets, hygiene rooms and personal care

BB104 sets toilet numbers by independence, not just headcount. Where pupils are less independent, it asks for "approximately one fitting for every 10" full-time equivalent pupil places. Where pupils are independent and aged five and above, it is "one fitting for every 20 pupils in total". Every setting needs "at least one accessible toilet for disabled pupils per floor".

Hygiene rooms are the bigger design driver. Non-ambulant special schools and Units need "one hygiene room for every 12" non-ambulant pupil places, each with a fixed hoist, changing bed, wash hand basin, accessible WC and space for assistants. Ambulant special schools and AP need one per setting. The guide sizes run from 7m2 for a changing bed and basin in a primary setting to 20m2 for a peninsular changing bed with an accessible WC and accessible shower. BB104 prefers "fewer well-placed rooms of the right size" to a larger number of undersized ones.

What does BB104 say about existing buildings and extensions?

Most SEND capacity projects are extensions, remodels or conversions, and BB104 has specific rules for them.

When places are added to an existing special school or AP, the base areas such as the main hall, head's office and staff room usually exist already, so only the area per pupil place for the additional places is needed. That covers the extra classrooms and the support spaces that go with them.

Existing rooms rarely fit the zones neatly. Where a room is below the recommended minimum, BB104 says group sizes should be reduced or spaces combined. Where rooms are oversized, it allows a fit factor on the recommended areas, and "Where the net and gross areas are more than 15% higher than the recommended figures, remodelling may be more economical". In listed buildings the fit factor "may be as high as 25%, or perhaps more."

Two other situations come up often:

• Co-location with a mainstream school can share the kitchen and premises office, giving "an area reduction of up to around 75m2", but the special school's teaching area should stay the same.

• Supplementary area, such as a community counselling centre, needs its own money: "a dedicated, additional funding stream needs to be identified for any supplementary area unless that area already exists in appropriate accommodation."

The site: outdoor space for special schools

Part B of BB104 covers grounds. The net site area divides into outdoor PE, informal and social areas, and habitat areas. Its ranges are wider than in mainstream guidance because outdoor use varies so much between schools: horticulture is central to some curricula, team games to others.

Security comes first. Outdoor spaces should give good sightlines "with no hidden spaces", with noisy areas kept apart from quieter, sheltered ones. Where pupils are less mobile, BB104 asks for sheltered accessible routes, safe surfaces and wide paths, noting that "ADM and BS8300 require 1500mm and 1800mm for busy routes".

For AP, SRP and Units, BB104 gives broad guidance only, without area requirements, since these settings share or vary so widely in their outdoor provision.

The full BB104 text, including the area formulae for every setting in Annex A and site areas in Annex B, is in the Building Bulletin 104 PDF.

How we help schools apply BB104

Area guidance is a starting point for a brief. The test is a building that works for the pupils in it.

Grayling Thomas Architects was CIF bid writer, lead consultant and architect for Dorothy Goodman School in Hinckley, a £6.2 million new building for an existing special school, funded entirely through the Condition Improvement Fund. The building was completed in September 2018. It was the winner of Large Non-Residential Scheme of the Year 2019 (Procon Leicestershire) and was recognised at the RICS East Midlands Social Impact Awards 2020.

If you are planning new SEND places, extending a special school or testing whether an existing building can take a Unit or SRP, we can help you read BB104 against your own pupils, site and budget, and turn the area ranges into a brief that works. You can see our approach on the SEN school architecture page and our wider education and SEN architecture work.

To talk through a project, contact Grayling Thomas Architects.