A Class MA conversion refused on natural light

Class MA prior approval is refused on adequate natural light more often than on any other matter, and the reason is structural rather than bad luck: the right permits a change of use and nothing else, so a deep-plan office that cannot light its habitable rooms through the windows it already has has no remedy inside the right. The walkthrough below is illustrative rather than a real case. It follows a 1970s office building through a refusal, a redesign and a second submission.

The short version: 22 flats became 15, and the developer bought the difference back in unit size rather than unit count.

The building and the proposal

A four storey office block of about 1,400 square metres gross internal area, built in the mid 1970s on an edge-of-centre site. Concrete frame, central core with stair and lift, continuous ribbon glazing on the two street elevations and blank brick flanks on the other two where it once abutted neighbours. Floor to ceiling height of 2.6 metres. Vacant for a year after the last tenant left.

The initial proposal was 22 flats: a mix of studios and one bedroom units on each floor, arranged around the central core, with a double-loaded corridor. On the street elevations the units were shallow and worked well. On the flank elevations they had no windows at all, so the design pulled light from a small internal lightwell that had been cut through the building in the 1990s for a smoking area.

That lightwell was the whole problem, and everyone involved knew it before the submission went in.

The prior approval matters engaged

Class MA sits in Part 3 of Schedule 2 to the General Permitted Development Order 2015 and allows a change of use from Class E commercial, business and service use to Class C3 dwellings, subject to prior approval on a defined list of matters. For this building, five were live:

• Transport impacts, particularly safe site access.
• Contamination risks, given a former printing use on the ground floor.
• Flooding risks.
• The impact of noise from nearby commercial premises on the intended occupiers.
• The provision of adequate natural light in all habitable rooms of the dwellings.

Two others did not apply here: the building was not on ground floor in a conservation area, and the site was not in an area the authority had identified as important for general industry, storage and distribution or waste management.

The qualifying conditions were checked first. The building had been in Class E use for well over two years. It was not listed, not within the curtilage of a listed building, not a scheduled monument and not on a site of special scientific interest. The original version of Class MA also carried a floorspace cap and a requirement that the building be vacant for three continuous months before the application, both of which were altered by a 2024 amendment, so check the current text of Class MA before relying on either.

Paragraph W requires the application to be accompanied by floor plans indicating the dimensions and proposed use of each room, including the position and dimensions of windows, doors and walls. That requirement exists precisely so the authority can test the light claim, and an incomplete plan set is not a valid application.

The natural light evidence and what it showed

"Adequate natural light" is not defined in the Order, so the assessment falls back on recognised practice. The consultant reported average daylight factor against the older BRE figures and also ran the newer target illuminance approach, and stated both.

The results split the scheme into three groups. Eleven units on the street elevations passed comfortably. Four units with living space on the street and a bedroom looking into the lightwell were marginal. Seven units drew all their light from the lightwell, and three of those had bedrooms with no external window at all.

A habitable room with no window fails the test outright, and no modelling rescues it. The lightwell rooms failed for a different reason: the well was 3.2 metres across and four storeys deep, so the windows at the lower two levels saw almost no sky. A room can have a perfectly reasonable window and still fail where the obstruction in front of it is that severe. Detail on when a daylight and sunlight assessment is required and what it has to cover is the starting point for this kind of evidence, and PlanScope drafts the structure while flagging the modelled outputs that only the consultant can supply.

The noise position, by contrast, was straightforward. A late licence two doors away meant a Noise Impact Assessment with facade attenuation and mechanical ventilation, which the authority accepted by condition.

The council refused prior approval, citing inadequate natural light in the habitable rooms of seven units. It said nothing about the other 15.

Why new windows were not an option

The obvious answer on paper was to punch windows through the blank flank walls. It was not available.

Class MA permits the change of use. It does not permit building works to the external envelope. New window openings in a flank wall are development needing planning permission in their own right, and that application would be judged on ordinary policy grounds, including overlooking of the neighbouring site and the future development potential of that site. On this building the flank sat 1.5 metres from a boundary with a site allocated for redevelopment, so new habitable room windows there would have been resisted and, if granted, would have constrained the neighbour.

Enlarging the lightwell was possible in principle but would have meant removing floor slab area on every level, structural work to the frame, and a reduction in the very floorspace the scheme was converting. The cost ran past the value of the units it would have saved.

This is the trap in Class MA generally. The right is generous about use and silent about fabric, so buildings whose plan depth exceeds what daylight can reach are simply not convertible under it, however willing the developer is to spend money.

Redesigning around the light

The redesign started from the windows and worked inwards, which is the opposite of how the first scheme was drawn.

Three moves did the work:

1. The seven lightwell units were deleted as separate dwellings. Their floor area was absorbed into adjoining units on the street elevations, making dual aspect flats that ran from the street to the well, with the well providing secondary light to a kitchen rather than primary light to a bedroom.
2. The internal rooms that could not be lit became non-habitable: a utility, a store, a plant room and, on two floors, a shared residents' cycle store. Habitable rooms were kept within 6 metres of an external window.
3. Unit sizes went up rather than down. The scheme moved from 22 small units to 15 larger ones, all meeting the nationally described space standard, which applies to dwellings created under prior approval.

The arithmetic mattered to the client and it is worth stating plainly. Fifteen larger flats in a well-located building sold for more per square metre than 22 units with unlit bedrooms would have done, and the second scheme carried no planning risk on the light matter at all. The gross development value fell, but not by the third the unit count suggests.

The second submission and the outcome

The second Class MA application went in four months later with the same site, contamination and flood evidence, an updated noise report, and a room-by-room daylight schedule showing every habitable room against the target with its distance from the window and the obstruction angle in front of it.

The council approved it within the 56 day period, with conditions on the noise mitigation, the contamination verification and completion within three years of the approval date.

Two details from the process are worth carrying forward. The officer's report on the second application quoted the daylight schedule directly, which suggests the format of the evidence mattered as much as the content. And the deleted lightwell rooms were shown on the plans as non-habitable with their uses labelled, which removed any argument about whether they were bedrooms in disguise.

What to take from it

Test daylight before you fix the unit count. On any building deeper than about 12 metres between windows, the number of dwellings is set by the daylight, not by the floor area, and running the numbers at feasibility costs a fraction of a refused application.

Check whether your fix needs fabric changes. If it does, you are in a full planning application, and the Class MA programme advantage disappears.

Label non-habitable rooms honestly on the plans, with a use for each. Paragraph W requires the room uses and dimensions anyway, and a clean plan set closes off the suspicion that a store will become a bedroom after completion.

And accept the trade early. Fewer, larger, properly lit flats is usually the right answer on a deep-plan office, and a scheme that reaches it at feasibility stage saves four months. The walkthrough of a mixed use scheme with retail below flats shows the same tension in a different form, where the ground floor use rather than the plan depth sets the residential layout.

Browse the report types to see which documents a prior approval submission is likely to need, or enter your postcode on PlanScope to get a list for your own building.

This walkthrough is illustrative and does not describe a real building or scheme. It is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.