Daylight assessment when the neighbour refuses access

There is no right of entry to a neighbouring property for a daylight and sunlight assessment, so the assessment is run on assumed internal layouts and the assumptions are declared. That is the normal condition of this work rather than a defect in it. The Party Wall etc. Act 1996 gives access for works, not for surveys. The Access to Neighbouring Land Act 1992 allows a court to order access for preservation works, which a daylight study is not. A neighbour who says no is within their rights, and the assessment still has to be produced.

What distinguishes a defensible report from a weak one is not whether the layouts were measured. It is whether the assumptions are conservative, sourced and stated in a form that an officer and an inspector can test.

What can be established from outside

A careful external survey yields more than people assume, and it is the foundation of every assumption that follows.

From outside you can establish window positions in plan and elevation, cill and head heights above external ground level, window dimensions and glazing area, the number of storeys and the approximate floor-to-floor height, the depth of the building from front to back, the position of chimney breasts and party walls where they are expressed externally, the presence of rear outriggers or back additions, and whether windows are obscure-glazed, bricked up or serve a stair.

Measured survey by total station or laser scanner, flown photogrammetry where access permits, and the council's own planning records give you the envelope to a tolerance that is good enough for the geometry. The vertical sky component is calculated at the outside face of the window, so it needs no internal information at all. Loss of sunlight to a window, measured by annual and winter probable sunlight hours, is also external.

That is the useful point to grasp. Two of the three standard tests in the BRE guidance can be completed without entering the building. Only daylight distribution, the no-sky line test, needs to know where the room boundaries are.

Assumptions about room use and layout

So the assumption set is narrower than it first appears. You need room depth, room width, the position of internal walls and the use of the room.

The approach that holds up in practice is to assume the arrangement that produces the worst result for the proposed development, within the bounds of what the building could plausibly contain. If a shallow room would pass and a deep room would fail, assume the deep one. If the window could serve a bedroom or a bathroom, assume the bedroom, because bathrooms, toilets, stores, garages and circulation space are treated as less sensitive in the BRE guidance and a reviewer will suspect convenience if you assign them.

Room depth is usually taken from the building footprint. In a Victorian terrace the front room depth follows the bay and the spine wall, both externally legible. In a 1930s semi the front and rear room depths are typically similar and the plan form is repetitive across the street, so a neighbour's sold floor plan for an identical house tells you a great deal. In a purpose-built block the floor plates repeat, so one known layout establishes the pattern.

Where the use cannot be inferred at all, assess the window as a habitable room and say so. An officer will accept a worst-case assumption far more readily than an optimistic one.

Sources for the internal arrangement

Work through these in order, and record which one each room came from.

1. The local planning authority's online records. Previous applications for extensions, loft conversions and conversions to flats routinely contain existing floor plans, and they are public documents.
2. Building control records where available, which sometimes carry plans not submitted with a planning application.
3. Historic marketing material for the property, which often includes a floor plan with room labels. Use it as evidence of arrangement, not as a document to reproduce, because the plan is somebody's copyright.
4. The energy performance certificate register, which records total floor area and habitable room counts and can confirm whether a plan form is plausible.
5. The pattern established by neighbouring properties of identical type, where one layout is known and the housing is repetitive.
6. The external survey alone, with a worst-case plausible layout assumed.

Source one is undervalued. On a street of terraces it is common to find three or four existing floor plans in the planning record within a hundred metres, and together they tell you the plan form of the whole terrace.

Do not knock on the door twice. One polite written request for access, kept on file with the response or the absence of one, is both courteous and useful evidence that you tried.

Stating the assumptions so they survive scrutiny

A table of assumptions, room by room, is what turns guesswork into method. For every window assessed, record the property, the floor, the window reference, the assumed room use, the assumed depth and width, the source of that assumption and the test applied.

Then run the sensitivity. Where a result is close to the benchmark, recalculate with a shallower and a deeper room and report both. If the conclusion holds across the range, the assumption has stopped mattering and you can say so. If the conclusion flips, say that too, because an honest statement that one room's result depends on an unverifiable layout is far stronger than a single number presented with false confidence.

Where reflectance or glazing transmittance also has to be assumed, the same discipline applies, and the way those assumptions move the results is set out in reflectance assumptions and daylight results.

Keep the assumption table in the main body of the daylight and sunlight assessment, not in an appendix. Burying it reads as concealment.

How an authority treats an assumed layout

Officers are used to this. A development management officer with a planning consultant's report in front of them will usually accept assumed layouts where the report explains the attempt to obtain access, states the assumptions, and adopts the worst reasonable case.

Weight does shift, though. An assessment on assumed layouts carries less weight on a borderline room than one based on measured internal dimensions, and committee members notice the word assumed. Where a single room determines whether the scheme is acceptable, the pragmatic answer is to design that room out of the problem rather than to argue about its depth.

Appeal inspectors take the same approach. Where both sides have produced numbers, the inspector tends to focus on whether the method follows the BRE guidance and whether the assumptions were reasonable, rather than preferring measured data as a matter of principle. Assumed layouts have been accepted. Assumed layouts chosen to favour the appellant have not.

Sunlight and overshadowing to a neighbour's garden involve no internal assumptions at all, which is one reason an overshadowing study sometimes carries the amenity argument where the window results are contested.

When the neighbour's surveyor arrives with the real plan

This happens, usually late, and sometimes at committee. The neighbour obtains advice, measures their own rooms and produces results that differ from yours.

Three things to do. Check whether the difference comes from the layout or from something else: a different obstruction model, a different window reference or a different baseline. Compare the two assumption sets directly rather than the two conclusions. And recalculate with their measured layout, because if your conclusion survives that substitution, the argument ends there.

If the conclusion does not survive, say so and address the planning consequence. A report that quietly maintains a superseded figure loses credibility for every other number in it. How to handle the wider objection is covered in answering a neighbour's daylight objection.

One honest uncertainty worth naming. Room use can change. A study today is a bedroom next year, and no assessment can fix the sensitivity of a room for the lifetime of the development, which is part of why the BRE figures are guidance rather than standards.

Wording the caveat

The limitations statement should be short, specific and free of defensive padding. Something along these lines, adapted to the facts:

Internal access to the properties listed in the table was requested in writing on the date stated and was not granted. Internal layouts have therefore been assumed from the sources recorded against each room, and where no source was available a worst reasonable case has been adopted. Window positions, dimensions and cill heights were taken from a measured external survey. Vertical sky component and probable sunlight hours results are unaffected by internal layout. Daylight distribution results depend on the assumed layouts and a sensitivity range is reported for each room where the result falls within 10 per cent of the benchmark.

Avoid the two failure modes. A caveat so broad that it disclaims the conclusions invites the officer to disregard the whole report. A caveat buried in small print at the back invites the accusation that it was hidden. State it once, clearly, near the front.

This is general guidance, not legal advice. Daylight and sunlight assessment should be carried out by a suitably qualified consultant, and councils usually expect a named author for this work.

To see which daylight, sunlight and amenity documents a scheme is likely to need, enter your postcode on PlanScope and work through the guided questions. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.