Basement Extensions and Encroachment

Thursday, 19th December 2019 | by: William Weaver

This post was inspired by a question posed recently by a homeowner who was concerned about the neighbour’s new basement excavation and whether or not it would undermine her property.

She wanted to know what precautions could be taken to ensure that the neighbour would dig under the party wall only and stop as soon as they had enough room to construct the new basement wall and underpin the party wall.

I had to explain the realities of this sort of undertaking. The contractors work in a very confined space which is dark, hot and noisy and it is difficult to be precise on all measurements. However, an experienced contractor can observe the existing footings when they have exposed them use these as a guideline to work to.

The party wall surveyor does not inspect the works for encroachment. The building control officer or the designing engineer will check the underpinning but that won’t give a clue as to any encroachment.

My owner discussed the possibility of drilling down on their side of the wall. I had to point out that the practicalities of such a proposal were out of the question. They would have to empty their reception rooms, lift up their floor boards and drill down all the way along the wall. Then if they were to hit concrete, they would know that there was encroachment but not how much. Furthermore, if the concrete is already there then it is too late.

The message is this, once the basement is excavated and constructed any encroachment will not come to light until the neighbour starts digging their own basement.

This led the discussion onto a case I dealt with in West London, in a very high value area densely populated with rows of Victorian terraced houses. All on shallow foundations with lime mortar masonry. In other words, expensive and fragile buildings.

The neighbour had bought her house with a basement already constructed a few years earlier. My appointing owner proposed a basement very similar to their neighbour’s basement.

The neighbour was delighted to hear that she would be receiving a windfall because my owner intended to make use of her basement wall and would therefore make a financial contribution to reflect the benefit.  The neighbour was looking forward to a year’s worth of school fees, perhaps more.

As the excavation progressed it became apparent that the neighbour’s contractors had undermined and encroached massively onto my appointing owner’s land and that now there were huge lumps of concrete filling up the space where my appointing owner wanted a basement.

The encroachment presented a serious problem. In some areas the encroachment was up to a metre inboard of the line of the party wall. If the excess was cut off using pneumatic drills or Kango type breakers the work would violently shake both houses causing extensive damage.

The solution was to cut the excess off with a wire saw. This is a powered cutting device that has three or more pulley wheels fixed to the concrete. A diamond encrusted wire is stretched between the pulleys and driven round by a motor creating, in effect, a giant cheese wire that cuts off the excess concrete.

A further complication was that once the concrete has been cut off it has to be lowered to the ground gently and the pieces are too large to remove from the basement. It can’t be broken up using drills, Kango breakers etc. co before the cut takes place, further ground has to be dug away so that the lumps can be buried under the basement.

All of this extra work cost tens of thousands of pounds, which was offset against the payment for making use of the wall. So, our hapless lady next door didn’t get her school fees paid at all in the end.

There were a couple of further technical aspects that present a potential trap for the unwary.

There is an argument that an enclosure payment such as the one due in the above case should go to the previous owner (who carried out the work) rather than the current owner who bought the property with an existing basement.

The neighbour in this case didn’t countenance this argument and insisted that she’d bought the property as is and had paid for the wall therefore should benefit from any windfall. This did however mean that she couldn’t then argue it wasn’t her responsibility when the encroachment was discovered.  She may have had a claim against the previous owner or the contractors but that is outside the scope of this post.

There was a further argument regarding the position of the “cut”. The argument runs:

1) If the cut is made at the toe of the corbelled brickwork then it is works of “repair” at the cost of the neighbour.

2) If the cut is beyond the toe of the brickwork – to make a thinner wall – then this is work to the wall to be carried out at the building owner’s expense not at the expense of the neighbour. This would have cost the building owner a lot of money.

There was a final complication, in that if the concrete was cut off and the resulting wall too thin would the engineers deem it safe and fit for purpose? In this instance there wasn’t a problem, but it is something to be beware of.

In summary, when envisaging a basement dig out, or having to endure a neighbour’s basement dig out, you are strongly advised to appoint a party wall surveyor who has some experience in the matter, who can advise on remedies in the event of encroachment, security payments, compensation and payments for enclosure on an existing wall.  The cost of the party wall surveyor is very minor in relation to the cost of the works.

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