Modelling a neighbour's room you cannot get inside

An assumed room layout in a daylight assessment is defensible as long as the assumptions are stated, sourced and tested. Access to a neighbouring property is a courtesy, not a right, and refusals are common on contested schemes. The useful first step is to work out which of your results actually depend on the interior, because two of the three standard measures in BRE 209 do not.

What you can calculate without going inside

Vertical sky component is measured at the centre of the outside face of the window. Annual probable sunlight hours are also assessed at the window, from the external geometry and the obstruction. Neither needs room depth, window position within the room, or furniture.

Daylight distribution is the exception. The no-sky line divides the part of the working plane that can see sky from the part that cannot, and it needs the room depth, the window head height, the working plane height and the room's extent to left and right. Internal illuminance calculations under BS EN 17037 need all of that plus surface reflectances.

So the honest framing in a report is narrower than "we had no access". It is that vertical sky component and sunlight results are unaffected, and that daylight distribution results rest on an assumed layout set out in a schedule. Reviewers should check that the report draws that distinction instead of applying a blanket caveat.

Deriving room depth from outside the building

External evidence is better than most people expect, and the sources stack up.

• A measured survey of the external envelope gives building depth, party wall positions, projections and the position of every opening. Where a rear addition exists, the depth of the main range is usually obvious from the step in plan.
• The council's own planning register often holds existing floor plans for the neighbouring property, submitted with an earlier extension, loft conversion or change of use application. These are public documents and they are the single most useful source available.
• Terraced and semi-detached stock repeats. Where the subject property sits in a run of identical houses, a plan form established from an accessible example of the same type is strong evidence, provided you say which property it came from.
• Chimney breasts, soil stacks, extract terminals and window proportions indicate internal subdivision. A narrow obscure-glazed opening with a terminal beside it is a bathroom.
• Historic maps and the building's construction date narrow the plan form, because plan types follow period conventions closely.

Record the source against each assumption. A schedule that says "room depth 4.2 m, taken from approved plans on application reference held by the council" is testable. One that says "assumed typical" is not.

Window head heights, cill levels and floor positions

The geometry that drives the no-sky line is the window head height above the working plane, so three figures have to be fixed: internal floor level, cill height and head height.

External survey gives the cill and head of the opening in site datum. Internal floor level is then inferred, commonly from the external threshold, the damp proof course position, or the relationship between the cill and the floor in a comparable property of the same type. Reveal depth and frame thickness reduce the effective aperture and should be measured rather than guessed, because on a nineteenth century sash in a thick wall the difference is material.

The working plane sits at 0.85 m above internal floor level for a residential room, so an error of 150 mm in the assumed floor level moves the no-sky line noticeably in a deep room. Say what you assumed and why.

Estimating room use from external clues

Room use matters because the weight given to a result depends on it. BRE 209 treats living rooms, kitchens and bedrooms as the rooms where daylight matters most, with bathrooms, lavatories, hallways, stores and garages less sensitive.

From outside, the reliable indicators are glazing type, opening size and position, the presence of extract terminals, curtain and blind patterns visible from the street or from your own site, and the position in plan relative to the rest of the house. The unreliable indicator is current use, since rooms change. A ground floor front room in a terrace may be a bedroom today and a living room next year, so the sensible assumption is the more sensitive use.

Where a window serves a room that may be subdivided or lit by a second opening, the analysis changes substantially, which is covered in our note on when a neighbour's room is served by two windows. The same reasoning applies in reverse to non-residential sensitive uses, as with the bedrooms discussed in our piece on daylight to hotel and care home bedrooms.

Stating assumptions so they can be tested

An assumption schedule belongs in the main body of the report, not in an appendix. For each affected window it should give the property and window reference, the assumed room use, the assumed room depth and width, the assumed internal floor level, the cill and head heights, the source for each figure, and a confidence note.

Then record the access attempts: the dates, the method of contact, what was offered, and the response. A letter sent by recorded post offering an inspection at the neighbour's convenience, followed by a reminder, is the evidence that your assumptions were a last resort rather than a shortcut.

Where you had access to some properties and not others, say which. A mixed basis is normal and transparent. A report that implies internal surveys throughout, when half the layouts were assumed, is the kind of thing that unravels at an inquiry.

Sensitivity testing rather than a single assumed case

One assumed layout gives one answer. A sensitivity test gives the range, and that is what makes assumed geometry acceptable to a decision maker.

The approach that works: model the central case from your best evidence, then re-run with a deeper room and a shallower room at the limits of what the external evidence allows, and with the floor level raised and lowered within its plausible band. Report the spread.

If the conclusion holds across the range, say so and the point is closed. If it flips between the shallow and deep cases, that is the finding, and the report should present it as such rather than selecting the convenient end.

How officers and inspectors treat assumed layouts

In practice, reasonable and clearly evidenced assumptions are accepted. A case officer weighing amenity needs a basis for judgement, and a transparent assumption with a sensitivity range gives them one. A neighbour who declined access is in a weak position to argue that the assumed depth is wrong, unless they are willing to provide the plan or the access that would settle it.

What attracts criticism is selective assumption. Where every assumed figure happens to favour the applicant, the pattern is visible across a schedule of twenty windows, and it damages the whole report. I would rather submit an assessment that shows two windows failing on an assumed layout, with the uncertainty explained, than one engineered to pass.

Keep in mind that this is planning amenity, separate from any civil right to light the neighbour may hold, and nothing in the planning assessment resolves that private claim.

Where assumptions remain unresolved, propose a condition requiring verification once access is available, or an undertaking to carry out mitigation if the verified layout produces a different result. That is a workable landing point and officers accept it more often than applicants expect. The surrounding report structure is set out on the Daylight and Sunlight Assessment page, and single-storey cases where geometry dominates are covered in our note on daylight and a single-storey rear extension.

Modelling of this kind needs a suitably qualified specialist, and the professional directory is the route to a reviewer who can sign off the method as well as the arithmetic. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.