Explosives safeguarding and population density

Licensed explosives sites have consultation zones around them, and inside those zones the thing that decides whether your scheme is acceptable is not its height, its design or its traffic. It is how many people it would put on the land and how vulnerable they would be. Population density is the controlling measure, and a scheme that fails on it cannot usually be mitigated into acceptability.

This applies to civil explosives storage licensed under the explosives regulations and to military explosives storage areas protected by a safeguarding direction. Both produce zones. They are administered by different bodies and they work in slightly different ways.

Why explosives sites are safeguarded

The risk being managed is a rare event with severe consequences. An incident at an explosives store produces blast, fragments and debris over a radius that depends on the quantity and type of material held, and the protective approach in the United Kingdom is separation distance rather than engineering the surrounding buildings.

That is why the zones are drawn at all. The separation distances associated with a licensed quantity of explosives are calculated to keep the number of people exposed within acceptable limits, and development that brings more people inside those distances erodes the protection the licence was granted on.

The corollary is important for landowners. The zone exists because of the licensed site, and if the licence changes or the site closes the zone can change with it. It is not a permanent designation in the way a conservation area is.

How the zones are structured

Both regimes use a set of nested zones rather than a single line. The inner zone is closest to the store and attracts the most restrictive view. The middle and outer zones progressively relax, and outside the outermost boundary no consultation is triggered at all.

What changes between zones is not whether development is possible but which types of development are acceptable. A small industrial unit occupied during working hours may be acceptable in a zone where a school would not be. The same building with a different use can produce a different answer.

The zone boundaries are not usually published as a public map layer in the way flood zones are. The local planning authority holds them, and they appear as a consultation trigger on the authority's system when an application is registered. If you want to know before you buy, ask the authority in writing.

Population density as the controlling measure

The assessment turns on two variables: how many people the development would place in the zone, and how sensitive those people are.

Sensitivity reflects the ability of occupants to be aware of a hazard and to respond to it. Housing occupied at night sits higher than a warehouse occupied by trained staff during the day. Schools, nurseries, care homes, hospitals and anything serving people with limited mobility sit higher still. Large gatherings, whether or not the building is permanently occupied, are treated as sensitive because of the number exposed at once.

That produces a matrix rather than a rule. A modest scheme of a sensitive type can be more problematic than a much larger scheme of an insensitive one.

Those are patterns rather than rules, and the actual advice depends on the zone, the quantity licensed and the numbers your scheme would introduce. Get the advice for your site rather than reasoning from the table.

Development types most affected

Residential is the obvious one, and the one that generates most of the disputes, because explosives sites are frequently in remote or edge-of-settlement locations where housing pressure is now high.

Less obvious cases catch people out. A change of use within a single use class can alter the population profile substantially, so an application that looks procedurally trivial can attract a strong response. Intensification of an existing use is caught for the same reason: adding a night shift to a building already inside a zone changes who is present and when.

Temporary uses are caught too. Festivals, markets and events that bring large numbers into a zone for a weekend are assessed on the number exposed rather than on the permanence of the structure.

And outdoor uses count. Population density is about people on land, not about floorspace, so a sports pitch, a campsite or a car park can be as relevant as a building.

Who is consulted and on what basis

For licensed civil explosives sites, the health and safety regulator is the consultee, and it gives advice on whether it advises against the development. The advice is not a veto. It is a material consideration of substantial weight, and an authority minded to grant permission against that advice is expected to notify the regulator before issuing the decision.

For military explosives storage areas, the safeguarding direction requires consultation with the defence authority, and the direction can restrict the grant of permission where an objection is maintained.

In both cases the consultee is answering a question about people and distance, not about planning merit. Arguments about housing need, design quality or the absence of alternative sites do not move the technical advice, though they may affect the weight the decision-maker gives it.

Provide the numbers the consultee needs with the application: the number of dwellings or the occupancy of the building, the hours of occupation, the nature of the use, and the site area within the zone. Applications that omit these generate a holding objection and a delay. That is one reason to establish the constraint at the earliest stage, in the same exercise as checking a site for constraints before you make an offer.

Mitigation and layout responses

There is far less scope for mitigation here than on most constraints, and saying so early saves months of testing options that will not work.

What can work is layout. Moving the built form to the part of the site furthest from the store, reducing the number of units inside the zone while keeping the total, placing car parking and open space in the zone rather than dwellings, and locating any sensitive use outside the zone entirely are all real responses. On a large site those moves can resolve the objection completely.

What does not work is construction. Blast-resistant glazing and structural strengthening are not usually accepted as answers to a population density objection, because the assessment is about the number of people exposed rather than the performance of a single building.

Phasing sometimes helps where the licensed quantity at the store is expected to reduce, but that depends on a third party's operations and is a weak basis for a planning case.

Engaging before the design is fixed

The sequence that works is simple and almost nobody follows it. Establish whether the site is in a zone before you buy. Ask the local planning authority for the zone extent and which zone your land falls in. Put an indicative population figure to the consultee informally, through the authority, before the layout is drawn. Then design to the answer.

Doing it the other way round produces a scheme that has to be redrawn after validation, and a delay measured in months. Where the constraint is fundamental, the honest conclusion may be that the site supports a different use from the one you had in mind, and reaching that conclusion at option stage rather than after a refusal is worth a great deal.

Set out the constraint and your response to it plainly in the Planning Statement rather than leaving the consultee to raise it. Officers respond well to an application that has already identified the problem and shown its working.

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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