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I’m worried my neighbour’s extension will flood my property

My neighbour refuses to share any plans and has not applied for planning permission

My question is in relation to the provision of guttering and water runoff into my property. My next-door neighbour is building an extensive extension at the back of his house. The extension is the full width of the garden to the boundary walls on either side. Internal guttering is visible at roof level. Flashing has been installed along the boundary wall along the length of the extension. The wall is about 1.8m high and the extension runs for about 8.5m. The flashing is at an angle and it appears that any water on the neighbour’s extension will flow down to the flashing and then into my property.

The neighbour refuses to share any plans and has not applied for planning permission so we have nothing to check against. What are the requirements to provide adequate guttering to prevent my property being flooded?

I would need more detail to fully understand the problem, such as the type of roof covering and exactly where the guttering and flashing for the extension roof is. In the absence of these facts, I have made lots of assumptions starting with the assumption that the boundary wall is legally a “party wall” and as such is in shared ownership between both neighbours. I also assume the proposed extension uses the party wall for support or abuts it closely and that the proposed extension will have a flat roof.

Regarding the flashing, I understand the boundary wall is 1.8m in height, so I assume the completed extension will have a finished roof height well above this, in the region of 3m from the ground. I assume the flashing is along the extension wall and dressed over the top of the boundary wall.


Flashing, in this scenario, is a thin sheet of metal, usually lead or zinc, to help waterproof the top of a wall or any steps or protrusions in the wall to direct water and prevent water from entering the gap between a boundary wall and the extension. I assume the gutter to the neighbour’s property is at roof level. It may overhang on to your site but drain to a rainwater downpipe ideally fixed to the rear wall of the new extension and discharge to a surface water gully at ground level in the next-door garden rather than terminating in your garden. Flashing in this scenario would be dressed on the top of the boundary wall to catch any small amount of rain on the section of the new extension wall above the boundary wall level and should not impede your wall too much, assuming it is properly dressed across the wall and properly secured to the extension wall. This in my view is an acceptable detail and represents a very low risk of water damage on your side.

Secondly, from my reading, the issue of compliance with planning and development legislation and building regulations is the overarching question. From my understanding, the proposed extension will be approximately 8.5m in length by an assumed conservative width of a terraced house of 5m, in other words 42.5sq m. By default, this is not exempted development and would require the neighbour to seek planning permission from the local authority. It is also worth noting that if the neighbour has an attic conversion or any other extension already constructed to their property, the permitted size of any extension under exempted development rules will be proportionately reduced.

If my assumption regarding the size of the extension is correct, a complaint for unauthorised development should be made to the local authority’s planning enforcement section. The identity of a complainant is confidential in line with section 35 of the Freedom of Information Act.

Given that the works have already commenced, the neighbour would be expected to apply for retention permission from the local authority. If such an application is lodged, you will have an opportunity to formally object to the works. I believe there is a substantial chance permission for these works will be refused.

Beyond the question of planning permission, it is relatively easy to design an extension in a manner that does not affect a neighbouring property in terms of rainwater runoff and/or surface water drainage. It is not standard practice for a proposed development to discharge water on to a neighbouring site without due consideration to drainage. Legal advice would be required to confirm if indiscriminate overflow from your neighbour’s roof on to your property would constitute trespass. It would also be assumed that the neighbour will require your permission for entry on to your property in order to fully complete any building works at or above the boundary wall, and also for maintenance purposes going forward.

It is worth considering the issue of support for the extension via the party wall. A survey and destructive opening up works may be required in order to determine whether or not the party wall is suitably designed to accommodate an extension built along its length. Foundations for boundary walls would not usually be designed or constructed to provide adequate support for an extension, so further investigations will be required in this regard in order to advise you appropriately. If I am correct in assuming the structure is legally a party wall, then you are legally entitled to receive notice and adequate information from the adjoining owner regarding proposed works affecting the party structure.

Finally, the risk of causing potential damage to any neighbouring property should be carefully assessed before any construction works are carried out on an adjacent site. Excavations and construction works can often have adverse effects on nearby structures, which is why we recommend a schedule of condition be carried out on your property in order to evidence any damage that may be caused to your home by the neighbour’s ongoing works. We usually recommend this be carried out at the earliest stage possible.

Either way, your neighbour needs to share drawings with you. Under the Land and Conveyancing Law Reform Act 2009, you are entitled to get a copy of the drawings showing the proposed treatment of the shared party boundary wall. Any work affecting the party wall cannot be started without the building owner notifying and obtaining consent from any affected neighbour.

There are a lot of unknowns in the question, and as such, we recommend you seek advice from your local chartered building surveyor, registered architect, or chartered engineer in order to fully understand the repercussions. Advice from a solicitor may also be required in order to clarify your rights, if any, with regard to the party wall, trespass, access and/or damage to your property as a result of the neighbour’s proposed extension.

Once the neighbour co-operates with you and their proposals are clear, I don’t think you should have any issue with a flashing across the top of the wall; any rainwater discharging to your side should be negligible. Good walls, well-dressed with neat flashing, make good neighbours.

Pat McGovern is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland

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