Party Wall
Apr 30, 2024

Encroaching on Basement Extensions

A neighbour might be concerned about new basement excavations next door and its potential impact on their property...

A neighbour might be concerned about new basement excavations next door and its potential impact on their property. 


Precautions need to be taken to ensure the neighbour excavates only under the party wall. This excavation would halt once adequate space was available for constructing a new basement wall. 


There are challenging working conditions for contractors in confined, dimly lit, and noisy spaces, making precise measurements difficult. However, experienced contractors can use existing footings as a guideline after exposure.

You will only know the extent of encroachment once you drill down your side of the wall.

Also, with Victorian buildings, the foundations tend to be shallow and made from lime mortar masonry. So, they are easy to damage.


Party wall surveyors do not ordinarily inspect for encroachment; this responsibility falls on building control officers or designing engineers during underpinning inspections. The discussion turned to the impracticality of drilling on one's side of the wall to detect encroachment due to logistical hurdles.


If the neighbour has already built a basement and you wish to build your own, you may encounter concrete spilling over to your side, which you would want to chip away without causing damage to the structural integrity of the neighbour’s basement. It can present a serious problem.


In London, there was a project involving basement construction at a terraced property. Despite initial optimism about financial benefits, unforeseen encroachment issues arose during excavation, leading to extensive concrete removal.

You might have to deal with a metre of concrete excess, which you would typically cut off with pneumatic drills or Kango breakers. However, the vibration can compromise structural integrity, resulting in health and safety issues.

Another solution would be to cut off the spillage with a powered wire saw, which has several pulleys fixed to the concrete. The wire is stretched between them and driving by a motor (similar to cutting away cheese). 

Safety concerns may arise regarding the resulting wall's thickness post-cut.

Once cut away, the concrete needs to be carefully lowered to the ground, but they may be too large to remove and so you bury them underground.

Enclosure Fees

If you already have a basement and your neighbour is connecting to yours, then you should be entitled to an enclosure payment, which is ordinarily half the cost of what you paid to build the party wall.

Further complexities include debates over entitlement to payments and the position of the cut, with implications for repair costs. So, if your existing basement has concrete spilled over and causes the neighbour to undertake extra work, this cost is set off against the enclosure fee total.

If the cut is made at the toe of the party wall, then this counts as repair work, which is deducted against the enclosure fee paid to the adjoining owner. However, if the cut extends beyond this to make a thinner wall, then this cost is borne by the building owner.


Appointing an experienced party wall surveyor familiar with basement projects will help you address encroachment issues, negotiate compensation, and ensuring fair payments. The cost of a party wall surveyor is relatively minor compared to the cost of overall project expenses.