If you have been using non-residential land or buildings without planning permission, you may have heard about the “10-year rule”. In some circumstances, a use that has continued for ten years can become lawful and immune from enforcement.

This article explains how the rule works for “other” material changes of use under section 171B(3) of the Town and Country Planning Act 1990, what the courts have said about it, and the common mistakes that lead to costly enforcement action.

What Does s171B(3) of the Town and Country Planning Act Say?

Section 171B(3) applies to any non-residential breach of planning control consisting of a material change of use (MCU) of land or buildings — except for the change of use of a building to a single dwellinghouse (which has its own rule).

In simple terms:

• If there has been an unauthorised non-residential material change of use;

• And that use has continued for 10 years;

• And no enforcement action has been taken within that period;

Then the use may become immune from enforcement.

However, the detail is critical. The courts have consistently emphasised that this is a strict factual test.

The Comparison Exercise: Then and Now

When an Enforcement Notice (EN) is issued, the key question is:

What was the use of the planning unit 10 years before the notice was issued, compared with the use on the date of issue?

You must identify:

• The correct planning unit;

• Its primary use;

• Whether there has been a material change;

• Whether that use continued for the full 10-year period.

Sometimes it is necessary to go back more than 10 years to establish when the breach first began.

What Is a “Material” Change of Use?

Not every change is “material”. The courts have made clear that materiality is a matter of fact and degree.

Issues often arise where:

• A non-residential use intensifies — when does growth become a new use?

• A non-residential use shifts within the same use class.

• An incidental use becomes a primary use.

• A site evolves into a mixed use.

If the character of the non-residential planning unit changes in planning terms, that may amount to a material change.

Mixed Uses and Additional Components

Mixed-use sites are a common source of difficulty.

In Beach v SSETR & Runnymede BC, the court confirmed that where an additional primary use (C) is added to an existing mixed use (A+B), the result is a change of the whole planning unit to a different mixed use (A+B+C).

You cannot treat the original uses as unaffected.

This means:

• The comparison is between A+B and A+B+C.

• If materially different, there has been a material change of use.

• The new mixed use must then continue for 10 years.

It does not matter how long A+B had existed previously.

This frequently catches landowners out — particularly where small commercial activities are added to established residential or agricultural uses.

Casual, Intermittent and Insignificant Uses

In Davies v SSE & South Herefordshire DC, the Court of Appeal held that no breach of planning control may arise where activity is merely “casual, intermittent and insignificant”.

This is important.

Occasional or trivial activity may not amount to development at all.

However, once an unauthorised Material Change of Use has occurred, the focus shifts to whether it continued for 10 years in a way that could have been enforced against.

The “Could the Council Have Enforced?” Test

A crucial principle was confirmed in Thurrock BC v SSE & Holding.

The court made clear:

A use only becomes lawful if it continued throughout the 10-year period to the extent that the local planning authority could have taken enforcement action at any time during that period.

If the use ceased during the 10 years — even temporarily — time may stop running.

The earlier case of Panton & Farmer v SSETR & Vale Horse DC is often cited in this context. The court explained that the idea of a “dormant” use only applies where lawfulness has already accrued. It does not help someone who is still trying to reach the 10-year threshold.

This distinction is frequently misunderstood.

What Counts as an Interruption?

An interruption may include:

• A genuine cessation of activity;

• A significant change in the character of the use;

• A change in the size or composition of the planning unit;

• An intervening material change of use.

The test is factual and case-specific.

Minor interruptions — such as:

• Short gaps during a change of ownership;

• Temporary illness;

• Slight fluctuations in activity levels —

will not usually be fatal.

But if enforcement action could not have been taken during the break, the clock may restart.

Temporary Planning Permission: A Hidden Trap

A particularly dangerous pitfall arises where temporary planning permission is granted during the 10-year period.

In Bailey v SSE, the court upheld enforcement action where a use continued after the expiry of a temporary permission. The appellant might have secured immunity before applying — but by applying for permission, the timeline was broken.

Once temporary permission expires:

• Continued use can become a new breach.

• The 10-year clock restarts.

Common Pitfalls

The most frequent problems associated with Section 171B(3) non-residential cases include:

1. Misidentifying the Planning Unit

Applicants underestimate the extent of the unit or fail to recognise that uses are interrelated.

2. Overlooking Mixed Use Evolution

Adding a storage yard, workshop, gym, or letting rooms to an existing use without appreciating that this changes the character of the whole unit.

3. Ignoring Gaps in Activity

Periods where the use genuinely stopped — even if only for months — can undermine the claim.

4. Applying for Temporary Permission

This can inadvertently reset the clock.

5. Weak Evidence

Ten-year immunity claims succeed or fail on evidence:

• Statutory declarations

• Business records

• Utility bills

• Photographs

• Tenancy agreements

Assertions are not enough.

Why Understanding This Matters

The financial stakes are often high for non-residential cases:

• Businesses forced to close;

• Retrospective refusals;

• Property sales collapsing;

• Mortgage difficulties;

• Criminal liability for non-compliance with an Enforcement Notice.

Equally, many landowners assume they are protected by the 10-year rule when they are not — or fail to realise they are protected and unnecessarily apply for permission.

A clear understanding of s171B(3) can:

• Protect existing operations;

• Support a lawful development certificate application;

• Strengthen a ground (d) appeal;

• Inform commercial negotiations.

A Matter of Fact and Degree

Ultimately, these cases turn on:

• The precise facts;

• The planning history;

• The character of the use;

• The quality of evidence.

The courts repeatedly describe these questions as matters of “fact and degree” — meaning small details can determine the outcome.

Take Advice Before Acting

The 10-year rule for other material changes of use is powerful — but technical.

Missteps such as:

• Misjudging materiality;

• Allowing a significant gap in use;

• Applying for temporary permission;

• Failing to assemble robust evidence —

can permanently undermine your position.

Before responding to enforcement action, submitting a lawful development certificate, or making a retrospective application, it is essential to obtain professional planning advice.

If you are concerned about an existing use or have received correspondence from the Council, seek advice early. The right strategy at the right time can make the difference between securing immunity and facing enforcement.

About me

Andrew Ransome is the planning director at ADP and is a chartered member of the RTPI, with over 22 years of town planning experience.

Andrew has extensive experience offering strategic planning solutions to challenging projects in both rural and urban settings. Follow him on Linkedin.

Get in touch