Checking a site for constraints before you make an offer

A site constraints check before purchase is desk work. Two days of it, done properly, will tell you whether the land you are about to bid on can carry the number of units your appraisal assumes. Most of the data is free and published. The part that costs you is the order you run it in, because some checks make every later check irrelevant, and running them the other way round is how small developers spend three weeks proving that a site they cannot build on has excellent broadband.

Start with the designations that remove the principle of development. Then move to the ones that add cost. Only then look at the ones that change your programme.

The order that kills deals fastest

Green Belt, National Park, SSSI, scheduled monument and Flood Zone 3b all sit in the first group. If any of them cover the developable area, the question stops being "how much will the evidence base cost" and becomes "is there a policy route at all". A site in Flood Zone 3b with housing on it is not a viability problem. It is a sequential test you will lose.

The second group costs money without closing the door. Conservation areas, listed buildings within influencing distance, Tree Preservation Orders, Air Quality Management Areas, coal referral areas, nutrient neutrality catchments. Each of these buys you a specialist report and, in the case of nutrient neutrality, potentially a mitigation payment that dwarfs the report fee.

The third group moves dates. Bat emergence surveys, great crested newt sampling windows, reptile survey seasons. They rarely stop a scheme. They routinely add four months to one.

Run the groups in that sequence and you can abandon a bad site on day one instead of day nineteen.

The national datasets that cost nothing

Four sources cover most of the first two groups.

The distinction that catches people is between a dataset that is authoritative and one that is indicative. Flood Zone 2 and 3 on the Flood Map for Planning are the policy zones, and the Environment Agency will be consulted against them. Priority habitat polygons on MAGIC are a modelled national inventory. An ecologist will confirm or reject them on the ground, and quite often rejects them.

Agricultural Land Classification is worth pulling early on any greenfield edge-of-settlement site. The published provisional mapping is coarse, at roughly 1:250,000, so it tells you whether best and most versatile land is plausible rather than whether it is present. If the answer is plausible, budget for a field survey by a soil scientist and read the answer on National Landscapes and the duty on decision makers if the site also sits in a designated landscape, because the two arguments tend to arrive together.

Data the authority holds and you have to ask for

Published national layers stop short of several things that decide applications.

1. The local validation checklist. Every authority publishes one, and it is the only document that tells you what will be demanded at registration beyond the national list.
2. Tree Preservation Orders not yet uploaded to the national dataset. Many are still held only as scanned orders in the council's own register.
3. Article 4 directions, including householder ones and C3 to C4 directions in university towns, with their precise boundary.
4. Contaminated land register entries and any historic landfill within 250 metres.
5. Critical drainage areas and the lead local flood authority's surface water records.
6. Section 106 obligations or restrictive conditions attached to the land by an earlier permission.

Numbers five and six are the ones that surprise buyers. A permission granted in 2009 on the adjoining field can carry an access condition that sterilises your only frontage. That will not appear on any national map. A request to the lead local flood authority will often turn up a critical drainage area the national mapping misses entirely.

Ask for all six in one written request before you exchange. Councils answer some of it free and charge for the rest, and the charge is trivial against the land price.

What only a visit will tell you

Desk data does not record ground conditions, informal access rights, fly-tipping, Japanese knotweed, the neighbour's septic tank outfall, or the fact that the "existing access" in the sales particulars is four metres wide with a wall on both sides and no visibility in either direction.

Walk the boundary. Photograph every tree that could be over 150mm in diameter, every ditch and every manhole. Look for made ground, because a level field on a valley side usually got that way with fill, and fill means a Phase 1 desk study turning into a Phase 2 intrusive investigation.

If there are mine entries in the area, order the coal mining report at this stage rather than later, and read what mine entries, permits and coalfield planning conditions mean for foundation design before you fix a build cost.

Seasonal constraints that move the programme

Ecology is the usual culprit. A preliminary ecological appraisal can be done in any month. The surveys it triggers cannot.

Bat emergence and re-entry surveys run from May to September, with May to August the reliable window. Great crested newt eDNA sampling runs from 15 April to 30 June and nothing will move that date. Reptile surveys need April to October with spring and early autumn the productive parts. Breeding bird surveys sit inside March to August.

Buy a site in October with a barn on it and you may not be able to submit a defensible application until the following August. That is not a reason to walk away. It is a reason to structure the purchase around it, which usually means an option or a conditional contract rather than an unconditional exchange.

Constraints that end a scheme

Some findings should stop the process the same afternoon.

Flood Zone 3b functional floodplain across the developable area, for anything other than water-compatible uses. A scheduled monument on the footprint, where consent is a separate national regime and refusal is common. Ancient woodland with no route to a 15 metre buffer. A site inside a hazardous substances consultation zone at a density the Health and Safety Executive will object to, which is worth checking against hazardous substances consultation zones before you model anything. And registered common land or a town or village green, where the deregistration route is slow, uncertain and sits entirely outside the planning system.

None of these are absolute in law. All of them change the probability of consent enough that the price you can responsibly pay drops below what the vendor will accept.

Building the findings into an offer

Translate each finding into one of three things: a cost line, a time line, or a condition of the contract.

Cost lines are the specialist reports and any mitigation. A Preliminary Ecological Appraisal, an arboricultural survey to BS 5837:2012, a Phase 1 desk study and a transport statement are the common four on a minor scheme, and each has a published consultant range you can use rather than guessing.

Time lines are the survey seasons and the determination period, eight weeks for a minor application and thirteen for a major, plus the realistic extension of time most authorities now ask for.

Contract conditions are where the unresolved risk goes. Subject to planning, subject to a satisfactory ground investigation, subject to the nutrient mitigation being available at a stated price. A vendor who refuses all three on a constrained site is telling you something about the site.

Keeping a dated record

Every dataset you check has a publication date and every one of them changes. Flood mapping is updated, conservation area boundaries are extended, Article 4 directions are made, nutrient catchments have been added more than once.

Screenshot each check with the date visible, save it to the project file, and note which officer told you what and when. If the application is refused two years later and you end up at appeal, that record is the difference between an argument and an assertion. It also protects you when a funder asks why you paid what you paid.

The report list that falls out of a constraints check is usually longer than the one in the sales particulars. You can see the likely bundle for a given site by entering the postcode on PlanScope or by reading through all 66 report types by category first.

This is general guidance rather than 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.