Optional technical standards: M4(2), M4(3) and space
M4(2), M4(3), the nationally described space standard and the tighter water standard apply only through an adopted local plan policy. What that policy must show and how the conditions work.
Optional technical housing standards are the small set of requirements a council can apply to new homes only if its adopted local plan says so, and only where the evidence supports it. Three matter in practice: the Part M accessibility categories M4(2) and M4(3), the nationally described space standard, and the tighter water efficiency requirement in Part G. Everything else that used to sit in local housing standards documents was swept away by the housing standards review in 2015.
For an architect, the practical question is simple. Does this authority's plan apply them, and to which units?
Why these standards are optional
Before 2015 a council could set its own standards for internal space, accessibility, water use and sustainability through local policy, and the resulting patchwork made national comparison impossible. The housing standards review consolidated them. Most were withdrawn, a few were folded into the Building Regulations as optional requirements, and the space standard was published separately as a national document that councils may adopt.
The result is a two tier system. The Building Regulations set the mandatory baseline for every new home. A local plan policy, supported by evidence, can raise that baseline for homes in its area by requiring an optional requirement to be met.
That architecture matters because it decides who enforces what. The planning system applies the standard through a condition. Building control checks compliance against the optional requirement in the Approved Document.
What a local plan has to show
National guidance sets out what an authority needs before it can apply these standards. For accessibility, the plan should be based on evidence of the likely future need for housing for older and disabled people, the size and type of housing needed, the accessibility and adaptability of the existing stock, how the standard can be met on particular sites, and the effect on viability.
Site specific factors are part of that. Ground conditions, flood risk requiring raised floor levels, steeply sloping sites and the practicality of step free access all bear on whether M4(2) or M4(3) can reasonably be delivered on a given plot. A plan policy that applies a standard to 100 per cent of homes without addressing these questions is vulnerable at examination.
Policies should also allow a reasonable transitional period so that schemes already designed are not caught mid-stream. Check the wording of the adopted policy rather than the summary in the plan's supporting text.
The accessibility categories
Part M of the Building Regulations sets three categories for dwellings.
| Category | What it covers | How it applies |
|---|---|---|
| M4(1) | Visitable dwellings, the mandatory baseline | Applies unless a condition imposes a higher category |
| M4(2) | Accessible and adaptable dwellings | Optional requirement applied by planning condition where policy requires it |
| M4(3) | Wheelchair user dwellings | Optional requirement, in adaptable and accessible sub-categories |
M4(2) is the category most plans apply across the board. It covers step free approach, wider doorsets and circulation, a WC at entrance level, walls capable of taking grab rails, and services positioned so the home can be adapted later without structural work.
M4(3) splits in two. The wheelchair adaptable version requires a home capable of simple adaptation to meet the needs of a wheelchair user. The wheelchair accessible version requires a home ready for immediate occupation by a wheelchair user, and national guidance limits that version to dwellings where the local authority is responsible for allocating or nominating the occupier.
Get that distinction into the drawings early. The turning circles and the transfer spaces drive bathroom and bedroom dimensions, and retrofitting them after the unit mix is fixed usually costs a unit.
The space standard and its limits
The nationally described space standard sets gross internal areas by the number of bedrooms, bedspaces and storeys, along with minimum bedroom sizes of 7.5 m² for a single and 11.5 m² for a double, a built in storage allowance, and a floor to ceiling height of at least 2.3 m over 75 per cent of the gross internal area. The smallest unit it describes is a one bedroom, one person flat at 37 m².
Two limits are worth knowing. The standard deals with internal area only, so it says nothing about layout quality, aspect, outlook or private amenity space. And it applies only where a local plan policy adopts it, which means a scheme in one district can be lawful at a size that would be refused across the boundary.
Where the standard does apply, it applies to the unit as built. Design to it with a margin, because a 36.8 m² flat measured off the construction drawings is a problem at building control stage rather than a rounding difference.
How the standards appear as conditions
Optional requirements take effect through a planning condition that specifies the optional requirement to be met. The condition should identify the units by plot number and category, because a blanket condition requiring, say, 10 per cent of units to meet M4(3) without saying which ones creates an argument at discharge stage.
Check three things when a decision notice arrives:
- That the condition names the optional requirement rather than a local policy number alone.
- That the plot schedule matches the approved drawings.
- That the water efficiency condition, where applied, states the litres per person per day figure from the optional requirement in Part G.
Where a scheme cannot deliver the required category on particular plots, make the case at application stage with levels, sections and a site constraints plan in the Design and Access Statement rather than seeking to vary the condition afterwards.
Consequences for layout and cost
M4(2) is the standard that changes plans most. Wider corridors, a level threshold, an entrance level WC and a stair capable of taking a lift later all consume area, and the effect compounds in small units where the circulation is already a large share of the plan.
The space standard interacts with it. Applying both to a small flat can push a one bedroom unit well above the 37 m² figure once accessible circulation is added, which is a design outcome rather than a policy failure. Plan for it in the massing, and test a typical unit at scheme inception rather than after the block layout is fixed.
Where the national position may change
The government announced an intention to make M4(2) the mandatory baseline for new homes in England, with M4(1) applying by exception. Reforms of this kind move through consultation, a government response and then amending regulations with transitional arrangements, and the position as at 16 September 2026 should be confirmed against the current Approved Document and the published regulations before it is relied on. The planning law timeline is a place to follow changes of this kind.
Policy references move too. If you are citing national policy alongside the standards, note that the Framework's structure changed with the August 2026 edition, and citing the Framework by policy code avoids quoting a paragraph number that no longer exists.
Check the adopted policy, then build the standards into the first unit plan. Enter your postcode on PlanScope to see which supporting documents the application 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. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is the difference between M4(2) and M4(3)?
M4(2) covers accessible and adaptable dwellings, with step free approach, wider circulation, an entrance level WC and provision for later adaptation. M4(3) covers wheelchair user dwellings and splits into a wheelchair adaptable version and a wheelchair accessible version, the latter limited by national guidance to homes where the authority allocates or nominates the occupier.
Does the nationally described space standard apply everywhere?
No. It applies only where a local planning authority has adopted a policy requiring it, supported by evidence of need, viability and the effect on affordability. That is why the same flat layout can be acceptable in one district and refused across the boundary. Check the adopted plan policy rather than assuming a national minimum.
What are the minimum room sizes in the space standard?
The standard sets gross internal areas by bedrooms, bedspaces and storeys, with a single bedroom at 7.5 m² and a double at 11.5 m². It requires built in storage and a floor to ceiling height of at least 2.3 m over 75 per cent of the gross internal area. The smallest unit described is a one bedroom, one person flat at 37 m².
How are optional requirements enforced?
Through a planning condition that names the optional requirement, with building control then checking compliance against the relevant Approved Document. The condition should identify the plots and the category, because a condition requiring a percentage of units without saying which ones causes disputes when the developer applies to discharge it.
Can a site be exempt from an accessibility requirement?
National guidance expects policies to take account of site specific factors such as ground conditions, topography and raised floor levels required for flood risk. Make that case at application stage with levels, sections and a constraints plan in the design and access statement. Seeking relief afterwards by varying the condition is much harder.
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.