Budgeting a daylight and sunlight assessment

The cost of a daylight and sunlight assessment is driven by how much of the surrounding city you have to model, how many neighbouring rooms have to be assessed, and how many times the design changes afterwards. The report itself is a small part of it. Fee proposals vary widely for the same site because assessors make different assumptions about those three things, so a comparison is only meaningful once the scope is defined in the same terms.

Define the scope first and the quotes become comparable. Leave it vague and the lowest fee will be the one that assumed the least work.

What the assessment measures

Assessment in England follows the BRE guide to site layout planning for daylight and sunlight, and it answers two different questions.

The first is the effect on existing neighbours: the vertical sky component at their windows, the distribution of daylight within their rooms, annual probable sunlight hours where windows face within 90 degrees of due south, and overshadowing of gardens and amenity areas. Each has a numerical test, generally expressed as a retained proportion of the former value.

The second is the quality of the internal environment in the proposed units, which the current guidance addresses through illuminance based methods rather than the older room depth rules.

Those are separate exercises with separate costs, and a fee proposal covering only the first will not answer a planning officer asking about the second.

Modelling the surrounding context

Most of the cost sits here. The assessor builds a three dimensional model of the proposal and of every building whose windows might be affected, accurate enough that a window position is right to within a few centimetres.

The model can come from a topographical and measured building survey, from aerial survey data, from point cloud scanning, or from a combination. Accuracy costs money, and the required accuracy depends on how marginal the results will be. A scheme comfortably passing every test can tolerate approximation. One that will be argued at committee cannot.

The extent of the model is the decision that moves the fee most. Agree the study area with the local planning authority before commissioning: which neighbouring properties are in, which are excluded and why. An assessor who models two streets when the council wanted four has produced a report that has to be redone.

Getting hold of neighbouring floor plans

The tests on existing neighbours depend on the layout inside their homes: which rooms the windows serve, how deep those rooms are, and whether a room is a living room, bedroom or kitchen.

You will not have that information, and there is no register of it. The usual routes are planning archives for earlier applications on the neighbouring property, sales particulars, historic building control records, and in some cases writing to owners to ask for access.

Where none of that works, the assessor makes reasoned assumptions and states them. Assumed layouts are the most common line of attack on a daylight report, so the assumptions have to be conservative and explained. An assessment that assumes a generous room depth in the neighbour's favour is harder to attack than one that assumes the reverse.

Budget for the effort of obtaining plans. It is unglamorous work that takes real time, and assessors either price it or leave it out.

Overshadowing of gardens and amenity space

Gardens and communal amenity areas get their own test, based on how much of the space receives at least two hours of sunlight on 21 March. It is assessed separately from the window tests and needs the same context model, so scope it in at the start rather than adding it later.

On schemes with communal courtyards between blocks, this test frequently drives the massing more than the window tests do. A courtyard enclosed on four sides by six storey blocks will struggle, and no amount of reporting changes that. Test it at concept stage with the massing options.

Child play space and roof terraces raise the same question in London, where borough policies and the London Plan set expectations for amenity space quality alongside quantity.

Testing during design rather than after it

The cheapest daylight advice is the advice you take early. A massing study at concept stage, testing two or three options against the neighbouring windows, costs a fraction of a full assessment and changes the scheme while changing it is still free.

By contrast, a full assessment commissioned after the planning drawings are complete can only report the result. If the result is poor, the options are to redesign and rerun, both of which are chargeable, or to argue that the breaches are acceptable in context, which is a weaker position than having designed around them.

An illustrative sequence that works: massing option testing at concept, a baseline assessment once the massing is fixed, one confirmatory run after detailed design, and a final report for submission. Three engagements, each with a defined purpose, and no surprises at the end.

Assessing the proposed internal environment

Councils increasingly ask for assessment of daylight within the proposed units as well as the effect on neighbours, particularly on flatted schemes with deep plans, balconies above windows or single aspect north facing units.

The variables that drive results are predictable: room depth, window head height, balcony overhangs, external obstruction and internal surface reflectances. Testing them early is what stops a scheme from arriving at planning with a quarter of its units failing.

This part of the work is usually priced per unit type rather than per unit, so a scheme with six repeating typologies costs far less to assess than one with 30 bespoke layouts.

Reporting for committee and for appeal

The submitted report is not the end of the budget. Add the assessor's time for responding to consultation comments, for a rebuttal where an objector commissions their own analysis, for briefing the officer, and for attending committee.

At appeal the cost steps up again, because proof of evidence, rebuttal proof and appearance at a hearing or inquiry are each priced separately and each takes days rather than hours.

Two ways to control this. Ask for a schedule of rates for additional work at the outset rather than negotiating it under pressure. And get the assumptions agreed with the council's officer before submission, since most late cost comes from arguments about method rather than about results.

Where re-runs add cost

Every design change that moves a wall, a window head or a balcony invalidates the model. The changes that trigger a rerun are more numerous than designers expect, and a rerun is not merely pressing a button: the model has to be rebuilt, the tests rerun and the report rewritten.

Batch changes rather than sending them individually. One rerun covering five amendments costs a fraction of five reruns covering one each, and the same discipline applies to the evidence base generally, as set out in budgeting an acoustic evidence base for a noisy site and budgeting a contaminated land investigation.

Daylight and sunlight analysis is work councils expect from a suitably qualified author, and professional review of any draft is strongly advised. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

See PlanScope pricing for what it costs to draft the planning and supporting documents that sit alongside a specialist daylight assessment. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.