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Property Surveying Blog
December 14, 2015

Party Wall Construction

Written by Bradley Mackenzie
In this week’s Party Wall blog post we are going to be looking at the difference between a party wall and a wall built wholly on the building owner’s land.

We will start by putting this into perspective! Under the Party Wall etc Act 1996 there are two types of wall that a building owner can build:

The first is a wall built up to the boundary line, however remaining wholly on the building owner’s land.

The second is a party wall built astride the boundary line, meaning that the wall sits equally on the building owner’s land and adjoining owner’s land.

We have included a very basic example of these two different types of walls below, hopefully this will help illustrate when the two different type of walls are applicable.

Party Wall construction

A wall built up to the line of junction 


Party Wall Construction Astride

A party wall built astride the line of junction 

In our experience here at Berry Lodge Surveyors, the majority of extensions are normally built with the flank wall built wholly on the building owner’s land.

The intention of this blog post is to help identify the benefits of a Party Wall built astride the line of junction as in our experience we feel this can often be a better solution.

So let’s look at the benefits:

Benefit 1) By building a party wall astride the line of junction the building owner will benefit from a slightly larger extension. To put this into perspective; building regulations will likely require the outer walls of a new extension to be cavity type.

Cavity walls tend to be made up of two leafs of bricks or blocks with a cavity between them, the cavity usually being filled with some form of insulation.

This type of wall is inherently thick and usually ranges from 250mm to 300mm.

This means that by building the wall as a party wall, half of the thickness of the wall will sit on the adjoining owner’s land. The width of the building owner’s extension will therefore be in the region of 150mm larger, a small but valuable amount space!

Benefit 2) Assuming you live in a semi detached or terraced property you will already be separated from your neighbours by a party wall. Therefore, building a party wall will be in keeping with the original construction of the property.

Benefit 3) By building a party wall astride the line of junction you will actually be saving money on the construction costs of the wall.

How does that work you ask?

Well put quite simply, when the adjoining owner, both the current or future owner, intends to build a similar extension, rather than building another wall alongside the party wall, they will instead enclose directly upon the wall previously build by the building owner.

When they enclosure upon the wall they will then pay half the construction cost. This means that both the building owner and the adjoining save money as they are only paying for the construction of half a wall, as opposed to both paying the full cost of the wall.

The enclosure cost payable by the adjoining owner tends to be calculated by a few different ways; a quantity surveyor’s priced costing, using the Royal Institute of Chartered Surveyor’s Building Cost Information Service (also know as the BCIS) which takes into account 50 years of construction data or finally a contractor’s priced quote.

The key thing is to remember is that the costing should be transparent and impartially calculated; it is for that reason that here at Berry Lodge Surveyors we favour a quantity surveyor’s priced costing or a BCIS costing.

Benefit 4) The foundation will be a standard type, meaning that it sits equally on both the building owner’s property and the adjoining owner’s property.

Contractors generally prefer ‘standard’ forms of construction as it is quicker and cheaper.

The alternative would be an eccentric foundation which you can read more about on our other Party Wall blogs.

Benefit 5) The future construction issue.

Assuming the building owner builds a wall wholly on their land, up to the line of junction. The adjoining owner will later have to do the same when they undertake a similar extension in the future.

This will mean there are two walls built alongside each other. In practice this often means the adjoining owner faces a tougher construction project as there will be a number of points they will need to consider, these include:

  • Sectional hit and miss bay foundations
  • A weatherproofing flashing detail at roof level
  • Some form of weatherproofing seal at the junction of the two walls
  • And finally the adjoining owner will likely need to install a slip membrane between the building owner’s existing foundations and their proposed foundations.

In our experience here at Berry Lodge Surveyors the construction of a party wall not only has short term benefits, however it also avoids future construction issues and it is for this reason that we believe they are a better solution on semi detached and terraced properties.

Here at Berry Lodge we have dealt with all forms of walls being built up to, or astride the line of junction. Whether it be a party parapet wall at roof level, the flank wall of a front, side or rear extension or a subterranean basement wall, you can bet that we have probably dealt with a job where it has been applicable!

If you are planning on undertaking construction works that are covered by the Party Wall etc Act 1996, or your neighbour is planning on undertaking construction works covered by the Act, call our team of experienced and qualified surveyors now who will be happy to assist you with your Party Wall enquiry.