My neighbour wants to use our shared back wall to extend, should I let him?

Property Clinic: If he builds first, what happens if I want to extend later?

Photograph: iStock

Photograph: iStock


My next door neighbours have applied for planning permission to build a single storey extension to the rear of their house which is semi-detached with my house. They have asked me about removing the existing narrow concrete block wall, which is a party wall between our respective back gardens, for a distance of about five metres to accommodate their extension which they say will be built along the same boundary line.

I have only recently bought my house and it had been my intention to build a similar extension within a few years. However, as they have got their planning application in before me, I feel that my own proposals have been somewhat compromised. I would like to facilitate them as much as possible as I wish to retain a friendly relationship my neighbours. How do I deal with their request?

There are a number of aspects to your neighbours’ proposal that you need to consider, including the boundary, your neighbours’ planning application, construction details and rights relating to your own proposals.

The boundary between your respective properties is located along the centre of the existing party wall. This will not change unless there is a formal exchange of a defined area or areas of property. Therefore, any agreement you reach, in relation to the extent of your neighbours’ proposals, must be made in the context of this existing boundary line.

The fact that your neighbours have already submitted their application for planning permission should not compromise your proposals. Planning approval for their extension, if granted, does not confer a right to interfere with the party wall, or build on, or project across the wall, into your property without your consent. Planning application forms include a request for evidence of the consent of the relevant landowner if any part of the site of the proposed development is not in the ownership of the applicant. The removal and replacement of the party wall with a new wall constitutes an encroachment on your property. Your neighbours do not appear to have complied with this application requirement.

The construction of your neighbours’ extension will have implications for your proposals. You described the existing party wall as a narrow concrete block wall. It is likely therefore to be a 100mm (4 inch) block wall. This means that only 50mm of the wall thickness is on your property. If your neighbours’ proposals require the new wall to be centred on the boundary line, it is likely that, as an external wall with standard insulation, it will extend for up to 150-200mm on your property. A roof projection, comprising standard soffit, facia and eve gutter, could project the new extension by approximately a further 300-400mm into your property. This projection into your property would therefore seriously compromise your proposals by restricting the space available for your extension.

You state that you want to facilitate your neighbours. The best outcome, therefore, from your perspective, is that you share the wall of your neighbours’ extension to support your proposed extension. To do this I strongly advise that you have a formal agreement in place that will ensure that the new wall is a party wall and ensures your right to use the wall for support.

Therefore, you should inform your neighbours that you wish to facilitate their proposals, inform them of your own proposals and state that you wish to have a formal agreement accordingly in the interests of both parties. The agreement should acknowledge that the existing boundary line, along the centre of the existing 100mm wall, remains as the property boundary line.

In addition to providing for your right to support for your proposed extension, the agreement should provide for a minimum roof projection from your neighbours’ extension into your property. It should provide for the removal of this projection and the construction of an appropriate roof joint and valley in the event of the construction of your extension.

The agreement should specifically provide for the foundation and wall of your neighbours’ extension to have sufficient bearing strength to support your extension. An engineer’s certificate confirming the suitability of the foundation and wall to support your extension will be necessary.

This formal agreement, drafted by a solicitor with input from an engineer, will be a safeguard against your neighbours objecting to your use of the wall as a support for your extension when you decide to proceed with it. The agreement will be essential if your neighbours sell their house and the next owner is unwilling to cooperate with you.

As stated above, your neighbours cannot remove the existing party wall or construct a roof projection across the centre of this wall without your consent, irrespective of their planning approval. An arrangement, as outlined in the agreement suggested above, will maximise the space available for both extensions, and facilitate the proposals of both you and your neighbour.

Patrick Shine is a chartered geomatics surveyor, a chartered civil engineer and a member of the Society of Chartered Surveyors Ireland,

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