Daylight in a converted office under Class MA
Class MA prior approval can be refused on natural light alone, and the test covers every habitable room. How adequate light is assessed and how deep office floorplates fail it.
The Class MA daylight requirement is one of the few prior approval matters that can stop an office to residential conversion outright. Class MA of Part 3 of Schedule 2 to the General Permitted Development Order 2015 lets you change a Class E building to dwellings without a planning application, but the local planning authority can refuse prior approval on a defined list of matters, and one of them is the provision of adequate natural light in all habitable rooms of the proposed dwellings.
That wording does a lot of work. It applies to all habitable rooms, not most of them, and it applies to the scheme as drawn rather than to a promise of improvement. A floor plan showing a windowless bedroom at the core of a deep office floorplate is a refusal, and no amount of argument about the housing shortage changes that.
Natural light as a prior approval matter
Prior approval under Class MA is not a free pass. The authority considers a closed list of matters, and natural light sits alongside transport, contamination, flooding and noise from nearby commercial premises.
The application has to include floor plans showing the dimensions and proposed use of each room, together with the position and dimensions of windows. That requirement exists specifically so the authority can assess the light question, and an incomplete plan set is the quickest way to lose the 56 day determination period.
Two structural points about the regime. The authority cannot refuse on matters outside the list, so design quality, tenure and mix are not in play. But within the list its judgement is real, and a refusal on natural light leaves you with an appeal or a full planning application rather than a negotiation.
Check first whether Class MA is available at all. Article 4 directions withdrawing it are in force in a number of central and town centre areas, and the conditions attached to the class have been amended more than once. As at the date of writing, the earlier 1,500 square metre floorspace cap and the three month vacancy requirement have been removed, but the requirement for the building to have been in Class E use for a continuous period before the application remains. Verify the current text before relying on any of that.
What adequate natural light means in practice
The Order does not define adequate natural light, and it does not name a standard. In practice two approaches are used, and it is worth knowing which one your authority applies before you commission anything.
The first is average daylight factor, from the earlier British Standard on daylighting. The familiar targets are 2 per cent for a kitchen, 1.5 per cent for a living room and 1 per cent for a bedroom. These figures are still widely quoted in planning submissions and still widely understood by case officers.
The second is the approach in the current European standard adopted into British practice, which works from target illuminance achieved over a proportion of the reference plane for a proportion of daylight hours, rather than from a single factor. The Building Research Establishment's site layout guidance has been updated to sit alongside it.
| Approach | What it measures | Where it helps a conversion scheme |
|---|---|---|
| Average daylight factor | Internal illuminance as a fraction of unobstructed external illuminance | Familiar, quick, and the targets are widely recognised |
| Target illuminance under the current standard | Illuminance achieved across the room for a defined proportion of hours | More demanding in deep rooms, but better evidence for a marginal case |
| No-sky line | Proportion of the room with a view of the sky | Useful for showing which part of a deep room is effectively unlit |
Pick one, state which one and why, and report room by room. A report that mixes methods invites the officer to apply whichever is less favourable.
Deep plan floorplates and inner rooms
The problem with 1960s to 1990s office buildings is geometry. A floorplate 18 metres deep with a central core was designed for open plan desks under artificial light, and it subdivides badly into flats.
Three specific failures recur:
- Rooms more than about twice the window head height deep, where the back of the room falls below any sensible daylight target.
- Inner rooms reached only through another room, which fail on light and also raise an escape problem.
- Single aspect flats facing an internal courtyard or a close obstruction, where the room has a window but little sky view. The techniques for daylight to single aspect flats and how to fix them apply directly here.
The design responses are limited but real. Reduce the depth of each dwelling and accept fewer, larger units. Put non-habitable uses, such as bathrooms, utility space and storage, in the dark core. Use the full window head height by raising ceilings where the structure allows. And consider dual aspect layouts at corners even where that produces awkward unit sizes.
Note that the nationally described space standard applies to dwellings created under these permitted development rights, so shrinking rooms to improve the daylight factor has a floor below which you cannot go.
Rooms without a window
A habitable room with no window will not pass. There is no daylight factor to calculate, and no modelling will help.
The term habitable room is not defined in the Order. The working interpretation is that bedrooms, living rooms, dining rooms and kitchen-dining spaces are habitable, while bathrooms, WCs, utility rooms, storage and circulation are not. A kitchen that is purely a galley off a lit living space is usually treated as part of that space rather than as a separate room, but a separate enclosed windowless kitchen is a risk.
Two layouts get proposed repeatedly and both are weak. A bedroom borrowing light through a glazed internal partition from a lit living room is not a room with natural light. And a bedroom opening onto a glazed winter garden on the facade may work, but only where the glazing and depth are modelled and the room still meets the target.
If a layout depends on either, test it before submission and be ready to redraw.
Modelling to support a prior approval
For anything other than an obviously compliant scheme, commission a Daylight and Sunlight Assessment and submit it with the prior approval application. Councils usually expect a suitably qualified author for this work, and professional review is strongly advised before you rely on the output.
The report should contain a room-by-room schedule with the room use, the floor area, the window area and head height, the calculated result and a pass or fail against the stated target. It should state the external obstructions used in the model, because an assessment that ignores the building across a narrow light well is worthless.
Include the failures. A schedule showing that two rooms out of forty fall short, with the design change proposed for each, is far more persuasive than a report that quietly omits them. Officers read these reports looking for the rooms that are missing from the table.
Where the authority applies local plan amenity policy alongside the prior approval test, read it carefully. Approaches differ, and how local plan amenity policies use BRE 209 varies enough to change what counts as adequate.
Light wells, new openings and the limits of Class MA
Class MA covers building operations reasonably necessary to convert the building, which raises an obvious question: can you cut new windows?
The honest answer is that it depends on the works and the building, and that the boundary is not settled. Forming an opening in an existing external wall to light a room may be reasonably necessary for the conversion. Excavating a new light well, altering the external appearance substantially, or extending the building goes well beyond the class and needs a separate planning application.
Where new openings are central to making the daylight case work, the safer route is often a full planning application for the conversion rather than prior approval. You lose the certainty of the permitted development route and gain the ability to design the building properly, and on a deep plan building that trade is frequently worth making.
What to submit, and the other conditions to clear
Assemble the application in this order and the determination usually runs smoothly:
- Confirm the building's planning history, its use class and the period of that use, and check for an Article 4 direction.
- Prepare floor plans showing each room's use and dimensions, with window positions and dimensions marked.
- Commission the daylight assessment and resolve the failures by redesign before submission.
- Address the other prior approval matters with the relevant evidence, including contamination, flood risk, transport and noise from nearby commercial uses.
- Add a short Planning Statement that walks through each prior approval matter in turn and says where the evidence for it sits.
That last document is not required, but it shortens the officer's work considerably and reduces the chance of a request for further information inside the determination period.
Enter your postcode on PlanScope to see which supporting documents a Class MA prior approval application is likely to need.
This is general guidance, not legal advice. A Lawful Development Certificate is the only formal way to confirm that works are lawful, so check the current Order and the position with your local planning authority or a qualified professional before relying on any of it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can a council refuse Class MA prior approval on daylight grounds?
Yes. The provision of adequate natural light in all habitable rooms is one of the prior approval matters the authority may consider, and a scheme showing a windowless habitable room will be refused. The authority cannot refuse on matters outside the prescribed list, but within that list its judgement carries full weight.
What standard applies to adequate natural light under Class MA?
The Order does not name one. In practice authorities use either average daylight factor, with the familiar targets of 2 per cent for kitchens, 1.5 per cent for living rooms and 1 per cent for bedrooms, or the target illuminance approach in the current European standard adopted into British practice. Ask which the authority applies.
Which rooms count as habitable rooms?
The Order does not define the term. The working interpretation is that bedrooms, living rooms, dining rooms and kitchen-dining spaces are habitable, while bathrooms, WCs, utility rooms, storage and circulation are not. A separate enclosed windowless kitchen is a risk, whereas a galley off a lit living space is usually treated as part of that space.
Can I cut new windows under Class MA?
Class MA covers building operations reasonably necessary to convert the building, and forming an opening in an existing wall may fall within that. Excavating a new light well, substantially altering the external appearance or extending the building does not. Where new openings are central to the daylight case, a full planning application is safer.
Do space standards apply to a Class MA conversion?
Yes. Dwellings created under these permitted development rights must meet the nationally described space standard, so room sizes cannot be reduced indefinitely to improve a daylight result. That interaction between minimum areas and daylight targets is what makes deep office floorplates so difficult to subdivide.
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.