Is Connecting Two Flats Together with a Door Permitted Development?

Is It Permitted Development to Connect Two Flats Together with a Door?

When it comes to altering properties, especially flats, one of the most common questions that arises is whether internal works such as adding a door between two flats requires planning permission. At first glance this might seem like a straightforward alteration, but under UK planning law the answer depends on how the flats are used after the works are carried out. This guide explains the legal background, what the courts and councils consider “development”, and whether linking flats with a connecting door is treated as permitted development.

Understanding the Legal Definition of “Development”

The Town and Country Planning Act 1990 sets out what counts as development. In broad terms, there are two categories:

• Operational development: physical works such as building, engineering, or other structural alterations.
• Material change of use: when the way land or a building is used is altered in a way that is significant in planning terms. Most works to property will fall into one of these categories, meaning they either need planning permission or must be covered by permitted development rights.

Does Creating a Door Count as Operational Development?

Internal alterations are usually outside planning control

Adding an internal door between two flats is technically a building operation, but it is very minor. Planning legislation in England focuses primarily on external alterations. Internal works generally do not need planning permission unless the property is:

• a listed building, in which case Listed Building Consent is required for almost any alteration, or
• subject to a condition on a previous planning permission restricting internal layout. 

Therefore, the act of cutting a doorway through a party wall between two flats is unlikely on its own to be considered development that needs planning consent. Building Regulations will still apply Even if planning permission is not required, Building Regulations approval will almost certainly be needed. This is because the separating wall between flats is usually a fire resisting and sound insulating wall. Introducing a door will require careful design to meet fire safety and acoustic standards.

Does a Connecting Door Change the Use of the Flats?

The bigger question is not about the physical works, but about whether the two flats are still being used as separate dwellings afterwards.

When does a change of use occur?

In planning law, a dwellinghouse or flat is defined by its ability to provide the facilities necessary for day-to-day living:

• a kitchen,
• bathroom facilities,
• living space, and
• sleeping accommodation. If you alter two flats so that they operate as a single household unit, then the lawful use changes from “two dwellings” to “one dwelling”. That is a material change of use and requires planning permission.

What if both flats remain self-contained?

If both kitchens, bathrooms, and separate entrances remain in place, each flat can still function independently. In that scenario, the introduction of a connecting door does not remove the ability to use the flats as separate units. The planning use as two self-contained dwellings is retained. However, if in practice the occupants use the property as one large flat, the council may decide there has been a material change of use regardless of the physical layout.

Case Law and Council Practice

The “unit of occupation” test

Case law has long recognised the concept of a “unit of occupation”. If a property is being used by one household, even if it still has two kitchens, the planning unit is treated as one dwelling.

Local authority enforcement

Many councils, including London boroughs and authorities such as Barnet, Westminster or Harrow, have taken enforcement action where two flats have been merged without permission. Even if both flats were technically still capable of independent occupation, the way the property was actually being lived in was enough for the council to argue a change of use.

Permitted Development Rights and Flats

Why flats are different

Permitted development rights are a set of national rights that allow certain works without planning permission. They apply most widely to houses, but flats do not benefit from the same permitted development rights. For example, you cannot build an extension to a flat or alter the external appearance of a flat under permitted development rules – you would need full planning permission. Therefore, any structural works to flats, even minor ones such as replacement of windows, are scrutinised more closely by planning authorities.

Practical Scenarios

Scenario 1: Door installed, flats kept separate

Two flats are linked by a lockable internal door. Both still have their own kitchens, bathrooms, meters, and entrances. The occupier chooses to live across both, but either flat could be rented out or occupied independently at any time.

• Planning position: The lawful use remains two dwellings. No material change of use has occurred.
• Risk: Low, though some councils may ask questions if one flat appears vacant for a long period.

Scenario 2: Door installed, flats used as one home

The owner links the flats and removes one kitchen. The family occupies the entire space as a single dwelling.

• Planning position: The use has changed from two flats to one dwelling. This is a material change of use and requires planning permission.
• Risk: High, as councils could take enforcement action. 

Scenario 3: Sale or mortgage situation

When selling, buyers’ solicitors often check planning history. If two flats have effectively been merged into one, but no permission has been granted, this can cause delays or affect valuation. A lender may also require certainty that two lawful flats exist.

Certificates of Lawfulness

For anyone in doubt, one way to protect your position is to apply for a Lawful Development Certificate (LDC) from the council. This is not planning permission but a legal confirmation that what you propose (or what you already have) does not require permission.

• If you intend to keep both flats self-contained, you could apply for a proposed use LDC to confirm that creating a connecting door does not amount to development.
• If you have already carried out the works, you could apply for an existing use LDC to regularise the position. An LDC provides certainty for resale, refinancing, or future planning disputes.

Key Considerations for UK Flat Owners

• Internal alterations like adding a door are rarely development, unless the building is listed.
• The real issue is use: are there one or two dwellings after the works?
• Flats lack permitted development rights, so always check carefully before making alterations.
• Building Regulations still apply for fire safety and soundproofing.
• Lawful Development Certificates are the best way to obtain certainty from the council.

Conclusion

Installing a door between two flats is not, in itself, usually considered development requiring planning permission. The real planning question is whether the two flats remain separate, self-contained units or whether they are effectively merged into one dwelling.

If both flats keep their own kitchens, bathrooms, and services, the lawful use as two flats is preserved, and the council is unlikely to consider that planning permission is required. If, however, they are occupied as a single dwelling, this is a material change of use and would normally require permission.

For peace of mind – and especially to avoid future problems when selling or mortgaging – many owners apply for a Lawful Development Certificate to confirm the position. This provides legal certainty and avoids the risk of enforcement.

If you are considering linking two flats with a connecting door, it is wise to seek professional planning advice from 4D Planning before carrying out the work.