Can our neighbour use the boundary wall as part of his extension?

My wife and I live in our own semi-detached house. Our adjoining neighbour is planning to build a one storey extension to the rear of his house extending out into his garden by 5m.

His plans are to use the existing boundary wall dividing both properties as part of the extension. We have no problem with this proposed plan but we are concerned as to any implications if we ever want to sell our house in the future.

There’s something about the prospect of neighbours working on their house that induces a state of high alert. Works to a shared boundary or on a party wall can send stress levels into the stratosphere.

Planning and programming works to a shared structure calls for level heads. My advice is to always engage a specialist.

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In other jurisdictions there are party wall surveyors. These surveyors will be appointed on both sides of the divide and will formulate a party wall agreement. This system has its roots in an environment where neighbours may be less familiar with each other than perhaps we are used to.

But just as good fencing makes for good neighbourly relations, it is vitally important that some form of agreement is in place to maintain the status quo. Too often issues left to chance can give rise to misunderstanding and dispute.

You mention that your neighbour hopes to retain and build on the existing garden wall.

This could in theory reduce the amount of disruption on your side but this approach is not ideal for a number of reasons.

Dividing line

In the first instance the wall and its foundations, designed as a garden wall, may not have sufficient structural ability to support the proposed extension.

The construction of this historic wall is unlikely to be of adequate quality to provide sufficient separation to form a party wall between the proposed extension and any future extension to your property. Party walls must resist the transfer of sound and fire. This would mean that any future extension would need its own new external wall alongside the boundary wall, thus losing space on your side.

The ownership of the boundary wall to be built on may be an issue. If you own half the wall as is often, but not always the case, you will need to formally permit your neighbour to encroach across the dividing line. This agreement would be important should either property be sold in the future or should you or future buyers plan to build an extension.

A better solution for both parties would be to agree that the boundary wall will be fully removed allowing for its replacement with a new solid wall that will facilitate an extension to both properties. This can act as an external wall and be rendered and made watertight until such time that your property is extended. In the event that your property is ever extended the wall will be of such construction that it will provide an adequate party wall.

If you sell your property in the meantime the presence of a new wall constructed to modern standards and capable of facilitating your extension will be an advantage.

I suggest that you appoint a chartered building surveyor who is familiar with this type of work. Your surveyor should liaise with the designer of your neighbour’s extension. The construction and finishes of the wall, easements or agreements and the like should all be formalised in advance of work starting. All potential issues should be ironed out. A suitably worded party wall agreement should be entered into with your neighbour.

The extension next door, if carried out correctly, will enhance your property and will also demonstrate the potential of your property.

Arrangements for access, times that work is permitted, flashing details and finishes, your right to use the wall as a party structure and the like should all be included in the party wall agreement. The agreement should be placed with your title documents.

An experienced professional will be able to steer you through what can be a stressful process. It is best to leave nothing to chance. – Noel Larkin

Noel Larkin is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland scsi.ie