Section 11 of the Party Wall etc. Act 1996 deals with costs. While the building owner typically pays for the works they initiate, Section 11(11) creates an important exception: if the adjoining owner later benefits from certain work that was originally carried out solely at the building owner’s expense, the adjoining owner may have to contribute a due proportion of the cost.
This provision is most commonly discussed where a building owner constructs a party wall (or a wall at/near the boundary) and the adjoining owner later uses that wall for their own development.
1) What Section 11(11) says (and what it is trying to achieve)
Section 11(11) provides that:
- where work is carried out solely at the building owner’s expense, and
- the adjoining owner subsequently makes use of that work,
the adjoining owner shall pay a due proportion of the expenses incurred by the building owner.
The Act goes further: the contribution is not simply calculated based on the original invoice. Instead, for the purpose of calculating what is due, the adjoining owner is treated as having incurred expenses by reference to what the work would cost at the time the subsequent use is made.
That is significant because it recognises a practical reality: costs change over time. The “benefit” being taken by the adjoining owner is assessed against today’s price of that work, not yesterday’s.
2) What counts as “subsequent use”?
The key phrase is “use is subsequently made” by the adjoining owner. In practical terms, this usually means the adjoining owner later relies on, builds off, or incorporates the wall/work into their own construction.
Common examples include:
- building a new extension and using the existing wall as a party wall or support
- building up from an existing party wall constructed by the neighbour
- using a wall on the line of junction as part of a new structure
- cutting into or otherwise utilising the wall in a way that derives structural benefit
In short: if the adjoining owner gains a genuine construction benefit from work that the building owner paid for, Section 11(11) is designed to require a fair contribution.
3) “Due proportion” does not automatically mean 50%
A common assumption is that the adjoining owner must pay half. In many straightforward cases—particularly where the wall sits on the boundary and is used equally—surveyors often approach the due proportion on a 50/50 basis.
However, the Act deliberately uses the phrase “due proportion”, not “half”.
That wording matters because the right figure depends on:
- the nature and extent of the adjoining owner’s use
- whether the wall/work is used equally or only partly
- whether there are features that make one owner’s use greater than the other’s
- any physical factors affecting how “shared” the wall actually is
A “due proportion” is a fair proportion, assessed on the facts.
4) Why the Act looks at the cost “at the time of subsequent use”
Section 11(11) provides that, for this purpose, the adjoining owner is taken to have incurred expenses calculated by reference to what the cost of the work would be if carried out at the time the subsequent use is made.
Practically, this means:
- costs are assessed at current rates when the adjoining owner begins to benefit
- inflation and changes in construction costs are factored in
- the calculation is not tied to historical invoices that may no longer reflect real cost
This avoids unfairness where one owner paid for work years earlier at much lower rates, and the neighbour later takes advantage of that work when replacement cost would be significantly higher.
5) When might the due proportion be less than 50%?
In many typical “equal use” cases, 50% is a sensible starting point. But there are scenarios where a building owner might reasonably be entitled to less than half back (meaning the adjoining owner pays less than 50%). This usually comes down to the wall not being used equally, or the adjoining owner’s use not reflecting a simple half share.
Examples often considered include:
A) The wall is not truly “shared” in position
If the wall is not equidistant on the boundary line (for example, it sits materially on one owner’s land), that can influence what is “due” in the circumstances.
B) The wall required structural repair at the time of the adjoining owner’s use
If, at the time the adjoining owner begins using the wall, the wall requires structural repair or has a defect that affects its suitability, the “due proportion” may need to reflect:
- the actual benefit being taken, and/or
- the cost that would reasonably be attributable to bringing it up to a suitable standard
The underlying principle is fairness: the adjoining owner should contribute to the cost of the work they benefit from, but not in a way that ignores the wall’s condition or unequal circumstances.
6) How is the due proportion assessed in practice?
Where Section 11(11) is engaged, the calculation and apportionment is typically dealt with by:
- agreement between owners, or
- if disputed, by party wall surveyor(s) under the Act’s dispute procedures
Surveyors may consider:
- what exactly was constructed and at whose expense
- what part of that work is now being used
- what the equivalent work would cost at today’s rates
- how the wall sits relative to the boundary and the nature of use
- whether repairs or upgrades are needed at the time of use
- what is fair and proportionate in the circumstances
This is why the Act refers to a “due proportion”: it gives surveyors a framework to reach a fair result rather than applying an automatic percentage.
7) Key takeaway
Section 11(11) is essentially a fairness mechanism. If you paid for work that your neighbour later benefits from, the Act can require them to contribute a due proportion—assessed at the cost of the work at the time they make use of it. While 50/50 is often a practical starting point where use is equal, the Act is clear that the correct figure depends on the facts, and there are circumstances where the contribution may properly be less (or potentially more) than half.
Need advice on Section 11(11) contributions or cost apportionment?
Email mail@howorth.uk or call 07794 400 212 and explain:
- what work was done,
- when it was done,
- how the adjoining owner is now using it, and
- any relevant drawings or historic notices/awards you have.