Specifying a topographical survey for a planning application

Three omissions cause almost every repeat visit by a survey team: levels on an arbitrary datum, an extent that stops at the red line, and no invert levels in the chambers. Each is avoidable in a single sentence of the brief. A topographical survey specification planning brief that covers those three, plus the tree measurements, will serve the application, the drainage design and the construction drawings without anyone going back.

The survey is also the least costly piece of certainty you will buy on the whole project. Everything downstream of it, the sections, the finished floor levels, the flood assessment, the cut and fill, is derived from it, and an error at this stage propagates into every one of them.

What the survey has to show

Write the brief as a list of features rather than as a description of the site. Surveyors record what they are asked to record.

• The site boundary as occupied on the ground, with fence, wall and hedge lines identified separately, and their heights.
• Spot levels across the site at a stated grid spacing, tightened around changes of slope, with contours at a stated interval.
• Existing buildings in plan, with ridge, eaves and finished floor levels.
• Adjoining buildings within the survey extent, with the positions and sill heights of windows facing the site.
• All hard surfaces, kerb lines, channel levels and the road centre line.
• Every manhole and inspection chamber, opened where safely possible, with cover level, invert level, pipe diameters and directions of flow.
• Utility surface features, including stop taps, gully positions, poles, lighting columns and cabinets.
• Trees, with stem position, stem diameter measured at 1.5 metres, canopy spread at four cardinal points and approximate height.
• Any watercourse, ditch or culvert within or adjacent to the site, with bed and bank levels.

That list takes five minutes to paste into an enquiry and removes most of the arguments later.

Datum and grid

Ask for levels related to Ordnance Datum Newlyn and coordinates on the Ordnance Survey National Grid. Say it explicitly, and ask the surveyor to state on the drawing how the datum was established and which benchmark or observation it derives from.

The reason is comparison. Published flood levels are given as metres above Ordnance Datum. Sewer records are too. Neighbouring surveys, if you ever need to compare, will be. A survey where the site datum was set at 100.00 on a convenient manhole cannot be compared with any of them, and a flood risk assessment cannot set a finished floor level against a design flood level without that tie.

On sites where a benchmark is not conveniently available, satellite positioning with a network correction service is the normal method and the surveyor should say so on the drawing.

How far past the red line

The default is to survey the ownership. The default is wrong on almost every site.

Extend to the far kerb of the adjoining highway, because the access design, the visibility splays and the levels at the new crossing all depend on it. Extend across every boundary to pick up the neighbour's ground levels and boundary treatment heights, because the 45-degree assessment, the overlooking analysis and the section drawings need them. Include the ground level and window positions at adjoining habitable rooms, because daylight and overlooking arguments are decided on those numbers.

And extend to any ditch, watercourse or outfall the site might drain to, with bed levels, because a drainage strategy that proposes discharge to a watercourse without a surveyed outfall level is an assumption dressed as a design.

None of this needs access to neighbouring land in most cases, since modern instruments will pick up levels and features from within the site. Where access is unavoidable, ask early and politely, and expect it to take time.

Trees, and the boundary between two professions

A topographical surveyor is not an arboriculturist, and neither should pretend otherwise. But the measurements the arboriculturist needs are measurements a survey team is well equipped to take.

Specify stem positions, stem diameters at 1.5 metres above ground, canopy spread measured at four cardinal points and approximate heights, and the result feeds straight into a tree survey prepared to the current standard. The arboriculturist then classifies the trees, assesses condition and calculates root protection areas.

Doing it the other way round, sending the arboriculturist first and then the survey team, means paying two mobilisation charges and reconciling two sets of tree positions that do not quite agree.

Accuracy, deliverables and formats

Agree three things before instruction.

The accuracy band. The recognised measured survey guidance sets out bands, and a band appropriate for detailed design is not the same as one for a feasibility plan. Say which you want and what tolerance you expect on levels.

The deliverables. As a minimum, a layered CAD file with levels as three-dimensional points or strings, a PDF at a stated scale, and the survey control details. Where laser scanning is used, agree whether the point cloud is included and in what format, because it is often priced separately.

The presentation. Contour interval, grid spacing, layer naming convention, north point, scale bar and the statement of datum and control. A CAD file with everything on layer zero will cost a designer half a day to untangle.

This is the point at which reading a fee proposal for scope and exclusions earns its keep, because chamber opening, traffic management for work in the highway and any return visit are the items most often excluded.

Reusing it downstream

A properly specified survey serves at least six later purposes: the existing and proposed sections, the finished floor levels in the flood assessment, the drainage falls and attenuation design, the cut and fill calculation, the daylight and overshadowing model where a three-dimensional context is needed, and the setting out on site.

Illustrative example: a three-unit infill on a sloping plot where the original survey covered only the ownership on a site datum. The flood assessment could not set floor levels, the drainage engineer had no invert to connect to, and the neighbours' ground levels were unknown for the section drawings. The resurvey cost more than the extra scope would have, and it delayed the submission by six weeks. That example is illustrative.

The omissions that trigger a resurvey

Watch for these, in roughly the order they cause trouble: an arbitrary datum, no invert levels, an extent that stops at the boundary, missing neighbour window positions, trees recorded as a canopy blob rather than as measured stems, a survey carried out before dense vegetation was cleared so half the site is inferred, and no record of the control used.

Add one more. A survey commissioned by a previous owner and handed over without the CAD file is a PDF, and a PDF is a picture.

You can check what your site triggers with the postcode on PlanScope, then use the survey to support the drainage, flood and tree work the constraints call for.

This 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.