Should We build the Side Wall of of Our New Extension Astride the Boundary?

by: The Party Wall Surveyor


I do have a query regarding a proposed one-storey side rear infill extension to a terraced property.. Is it best to build the side wall of the extension entirely on our side of the boundary line or is it in our interest to build it straddling the boundary line (making it a party structure) so that access for maintenance and construction is easier? What would be the pros and cons of both ways and are there any risks to be aware of?


The first thing to say is that unless there is already a shared garden wall astride the boundary (a party fence wall) you will require Adjoining Owner’s consent to build a new party wall.

Assuming that you get the consent there are a few advantages of building a new party wall:-

  • More internal floor area to your new extension
  • It will be easier for your neighbour to design a waterproof connection should they build a similar extension in the future – this may seem like an advantage to your neighbour but a poor detail between the 2 extensions could cause you problems in the future which could lead to disputes.
  • Possibility that your neighbour will contribute towards the costs of building the wall now or in the future (section 11(11) of the Act)

The common disadvantage that comes up is the soundproofing between your extension and any future extension built by your neighbour – you couldn’t stop them from enclosing a new party wall with their building. I don’t see that as a great disadvantage as the wall would be a cavity wall so would have greater sound proofing qualities than the existing wall between the 2 properties.

The only separation between the two properties along the boundary is a 1..8m fence. Does this not count as a party structure? If not and our neighbour does not give consent then is our only option to build up to the boundary line on our side? Is there no way to make it a party wall without his consent?! How then do we gain access on our neighbours land for the works to take place? Is this then a separate agreement not under the party wall act?


Fences are not classed as party structures so you would require your neighbour’s consent for a new party wall. You should serve your neighbour with a notice under section 1(2) indicating that you wish to build a new party wall – make it clear in a cover letter that they would not have to contribute to the cost of building the wall until such a time as they made use of it in the future (if they ever did).

They must reply within 14 days either giving their consent or stating that they wish the wall to be built wholly on your own land.

There is much debate amongst surveyors as to whether there is a right of access under the Act when a building owner builds a new wall up to the line of junction – the prevailing view currently is that there is (subject to 14 days written notice).