De minimis in planning judgments

De minimis in planning is a rule of thumb, not a statutory threshold. It borrows from the old maxim that the law does not concern itself with trifles, and in practice it means that some works are too small to count as development under section 55 of the Town and Country Planning Act 1990, or that a change in how land is used is too slight to be material. Councils accept the argument regularly. They also reject it, and there is no size, cost or percentage that settles the question in advance.

Where the idea comes from

Section 55(1) defines development as building, engineering, mining or other operations in, on, over or under land, or the making of any material change in the use of buildings or other land. Two routes out follow from that definition.

For operations, section 55(2)(a) takes out works of maintenance, improvement or other alteration that affect only the interior of a building, or that do not materially affect its external appearance. That subsection does most of the work people attribute to de minimis. Replacing a window frame with an identical one falls inside it. Replacing a sash window with a plastic casement, on an unlisted house outside a conservation area, usually does too, though the authority may take a different view where a design policy applies.

For uses, the word that matters is material. Whether a change of use is material is a question of fact and degree, judged on the planning consequences, and a change with no perceptible effect on anything an authority could reasonably care about is not material.

Neither route is a de minimis exemption in name. Both get called one.

Operational development and scale

There is no minimum size below which building operations stop being development. A small structure in a garden is still a building operation, and whether it needs permission usually turns on permitted development rights rather than on triviality.

What the courts have accepted is that some physical works are so slight that they do not amount to operations at all, or do not materially affect external appearance. Painting a wall. Replacing a roof tile. Fixing a house number. Nobody applies for permission for those, and no authority would entertain an application.

Between that and a rear extension lies a wide grey band. A satellite dish, an air source heat pump, a flue, a boundary wall, a hard standing, a set of steps. Every one of those has its own permitted development class with its own limits, and reaching for de minimis instead of checking the class is how people get into trouble. The class gives you conditions you can test. De minimis gives you an argument.

Change of use and the materiality question

Materiality is where the concept does real work, because the assessment really is one of degree.

An office where one employee occasionally sees a client has not changed use. The same office running an appointment-based clinic with twenty visitors a day probably has, because the planning consequences differ in traffic, parking, noise and hours.

Four factors tend to decide it.

• The character of the use, described in planning rather than commercial terms.
• The intensity, measured in trips, occupants, hours or deliveries.
• The effect on neighbouring amenity.
• Whether a separate planning unit has come into existence.

That last one catches people. Running a business from a spare bedroom is normally incidental to the use of the house. Converting the garage into a workshop with its own entrance, signage and customers creates a separate unit, and that is a material change whatever the floor area.

The places where a de minimis threshold really exists

A few regimes set an actual figure, and those are the only places the phrase carries legal weight.

The biodiversity exemption is the one most often misapplied. It is measured in habitat area, not site area, and both limbs have to be satisfied. A modest extension that takes out 30 square metres of lawn is outside it, and the scheme then needs a biodiversity gain plan approved before development begins, with the calculation done using the statutory metric by a competent person. The mechanics are set out on the Biodiversity Net Gain Assessment page, and the Environment Act 2021 regime expects that metric to be completed by someone competent to do it, with the authority approving the plan.

Why it is not a safe harbour

Three risks sit behind a de minimis assumption.

The first is that the authority disagrees. You have then carried out unauthorised development, and the remedy is a retrospective application or an enforcement notice, either of which costs more than asking first.

The second is enforcement time limits, which are shorter in practice than people assume and are changing. The 2023 legislation provides for a single ten-year period in England in place of the old four and ten year rules, and the commencement position should be checked against the current guidance rather than remembered. In the meantime, an unauthorised structure does not become lawful just because nobody has noticed it yet.

The third is conveyancing. A buyer's solicitor will ask for evidence that the works were lawful, and a seller's belief that the work was too small to matter is not evidence. That conversation happens at the worst possible moment, with a chain waiting.

For listed buildings there is no grey band at all. Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 catches any works affecting the character of the building as one of special architectural or historic interest, with no threshold written in, and unauthorised works are a criminal offence.

Cumulative works and the salami problem

Individually trivial works can add up. A window here, a rooflight there, a raised terrace, a widened opening. Each one defensible, the combination amounting to a material alteration of the building.

Authorities are alert to this and will assess the overall effect where works form part of a single scheme. Splitting a project into stages to stay under a threshold is not a strategy, it is a description of how enforcement cases begin.

The same applies to permitted development allowances, which are cumulative for most classes and count works carried out by previous owners. Check what has already been used before assuming an allowance is available.

Getting certainty instead of guessing

A Lawful Development Certificate is the only formal way to confirm that works are lawful. Section 192 of the 1990 Act covers proposed works and section 191 covers existing ones, and a certificate is a legal determination rather than an opinion.

1. Describe the works precisely, with drawings, since the certificate covers only what you described.
2. Set out the legal basis, whether that is a permitted development class, section 55(2) or the immateriality of a change of use.
3. Provide evidence, which for an existing use means dated photographs, invoices, utility bills or statutory declarations.
4. Expect eight weeks, and an appeal route to the Planning Inspectorate if it is refused.

A pre-application enquiry costs less and comes back faster, and it produces an officer's opinion rather than a determination. Useful for planning your approach. Not something a buyer's solicitor will accept.

As an illustrative example, a householder wants to convert a garage to a home office and let the space to a tenant as a therapy room two days a week. The building work may well be permitted development. The use is a different question entirely, and the answer turns on visitor numbers, parking and whether a separate planning unit has been created. Two questions, two answers, one application that should address both.

For a wider view of which documents an application of any type usually needs, browse the 66 report types by category, or enter your postcode on PlanScope to see what your site is likely to require.

This is general guidance, not legal advice, so 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.