Daylight assessment at outline stage with parameter plans

At outline stage you are not assessing a building, you are assessing the largest building the permission would allow. That is the whole point of a parameter plan, and it is also why daylight and sunlight work at outline stage so often produces numbers that frighten the applicant and the officer in equal measure. The assessment has to test the maximum envelope, because that is what the permission grants, and the maximum envelope is always worse for neighbours than the scheme anyone intends to build.

Getting this right means deciding early which parameters are fixed and which are reserved, then assessing the fixed ones honestly. A scheme that assesses an indicative layout and secures a parameter plan with more height in it has assessed the wrong thing.

Testing an envelope rather than a design

An outline application under article 5 of the Town and Country Planning (Development Management Procedure) (England) Order 2015 can reserve any of access, appearance, landscaping, layout and scale. In practice most large outline applications reserve appearance and landscaping, fix access, and control scale and layout through parameter plans showing development zones and maximum building heights.

For a daylight and sunlight assessment the only parameters that matter are the ones that define massing: the plan extent of each development zone, the maximum height within it, and any obligatory set-backs. Everything else, including window positions on the proposed buildings, is detail you do not have.

That gives you a clean assessment of effects on neighbours, using the methods in BRE 209 (2022), and a much weaker assessment of conditions inside the proposed buildings. Vertical sky component and the no-sky line can be tested against an envelope. Internal illuminance cannot, because you need rooms, windows and surface reflectances to model it.

Which parameters the assessment must fix

The parameter plans and the daylight report have to describe the same building. Where they diverge, an objector will find it.

• Development zone boundaries, dimensioned, with any zone of no development identified
• Maximum building height in each zone, stated as an absolute level above Ordnance datum rather than a storey count
• Minimum separation from the site boundary where that is being relied on
• Any mandatory upper floor set-back, with the level at which it applies
• Maximum and minimum building footprint where both are being controlled

Height stated in storeys is the most common weakness. A seven storey maximum with no floor-to-floor dimension and no datum gives a different envelope depending on who draws it, and at reserved matters the applicant will reasonably use the generous reading. State levels in metres above datum and give the storey count as commentary.

Where the site slopes, say explicitly whether the maximum height is measured from existing ground level at each point or from a fixed datum. The difference on a site falling 4m across its length is a whole storey at one end.

Worst case assumptions and what they cost you

Filling every development zone to its maximum height simultaneously produces a block that no architect would design and no viability appraisal would support. It is still the right test, because nothing in the permission would prevent it.

The consequences show up in the standard measures. Against the BRE guidance, a neighbouring window is normally considered to retain adequate daylight where the vertical sky component remains at least 27 per cent, or where it is not reduced to less than 0.8 times its former value. The daylight distribution test looks at whether the area of the room receiving direct sky light falls below 0.8 times its former area. For sunlight, a window within 90 degrees of due south should normally receive at least 25 per cent of annual probable sunlight hours with at least 5 per cent in the winter months, or retain 0.8 times the former value.

An envelope test commonly fails these where a detailed scheme would pass, and the honest response is to say so in the report and to quantify the gap. Run the envelope as the formal assessment, then run the indicative scheme as a sensitivity test, clearly labelled as illustrative and not forming part of the application. Officers find that helpful. Objectors will point out that the indicative scheme is not secured, and they are right, which is why the formal conclusion must rest on the envelope.

What to leave to reserved matters

Reserve the things that cannot be assessed meaningfully at outline and fix the things that can. Internal daylight to the proposed dwellings belongs at reserved matters, because it depends on window sizes, room depths, balcony positions and ceiling heights. The approach to assessing daylight in a proposed building's own rooms needs a design to assess.

Sun on ground in communal amenity space sits awkwardly between the two. An Overshadowing Study of the envelope will tell you whether half the amenity area can receive two hours of sun on 21 March, which is the BRE benchmark, and the answer depends mostly on massing rather than detail. That one is worth doing at outline, because if the envelope cannot deliver it, the parameters are wrong.

Trees, boundary treatment and the final position of windows on the proposed buildings can all wait. What cannot wait is the decision about whether the envelope is deliverable at all.

Conditions that link the two stages

The link between the outline assessment and the reserved matters submission is the part that is usually drafted badly. A condition saying that reserved matters shall be accompanied by a daylight and sunlight assessment is nearly useless, because it does not say what the assessment has to show.

Better wording identifies the outline assessment by its report reference and date, and requires the reserved matters submission to demonstrate that effects on named neighbouring properties are no worse than those reported, together with an assessment of internal daylight in the proposed dwellings against the standards current at that time. Name the properties. Where the reserved matters scheme would be worse at any window, the condition should require that to be identified and justified rather than simply prohibiting it, because an absolute prohibition invites an application to vary the condition later.

Check the condition against the six tests in Planning Practice Guidance on the use of planning conditions. A condition that requires a particular numerical outcome at reserved matters, where that outcome is not achievable within the parameters granted, is not reasonable and will not survive an appeal.

The risk of a result you cannot build to

There is a failure mode specific to parameter plans. The assessment reports acceptable effects on the envelope. Reserved matters then comes forward with a building that sits inside the envelope but concentrates its mass closer to one boundary, and the effect at a particular window is worse than the envelope run showed.

That sounds impossible and it is not. An envelope filled uniformly can produce a better vertical sky component at a specific window than a stepped building of the same overall extent, because what matters at one window is the obstruction within its 90 degree field of view. This is why the envelope assessment should report window by window rather than giving a scheme-wide pass rate, and why the worst affected windows should be identified by address and room use in the outline report.

An illustrative example. On a two zone parameter plan, Zone A at 18m above datum and Zone B at 24m, the envelope run shows a worst case retained vertical sky component of 22 per cent at a first floor rear window, which is below 27 per cent but above 0.8 times the former value of 28 per cent. At reserved matters the design pushes the taller element towards that boundary and the retained value drops to 19 per cent. The scheme complies with the parameter plan and fails the condition. Either the parameters needed a set-back, or the condition needed to be written against the parameters rather than against the reported numbers.

Presenting the assumptions

Write the assumptions down in one place at the front of the report, and keep them to a page. State the date and source of the survey used for the existing baseline, which neighbouring properties were modelled and why others were excluded, the window and room uses assumed in neighbouring buildings and the evidence for them, the surface reflectances used, and whether existing site structures were treated as present or demolished.

Room use in neighbouring properties is the assumption that gets challenged most often, because the BRE guidance treats living rooms, kitchens and bedrooms differently and the assessor usually cannot get inside. State that the uses were inferred from external observation and from published floor plans where available, and say what happens to the conclusion if a room assumed to be a bathroom is in fact a bedroom.

A short Daylight and Sunlight Assessment summary in the Planning Statement keeps the policy case and the technical case aligned, and saves the officer reading two hundred pages of tables to find the conclusion. Where neighbours have already written in, the approach to answering a daylight objection applies at outline stage just as it does on a detailed application.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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