Party Wall - excavating near the foundations

Oh,
Also we have our basement converted into a living area + gym. We did consider getting tanked as there are a few damp patches, but in the end decided not to (damp is not really too bad).

Could next door getting tanked cause issues for us? e.g. send additional moisture our way?

I'm not sure we would have any legal say in this either way however (unlike the party wall act)

looked into this recently. there is a rule around digging within 3 metres (I think) of a neighbours foundations which is probably the section they are referring to.

without digging a test pit on your land, with your permission I'm not sure how they can guarantee the drainage trench won't be deeper than your foundations.

I don't believe you can stop them doing it but you can ask for a survey of your property to be done, at their expense, and that would be used if there was any damage caused.

Assuming the basements are part of the original construction, unless the ground level changes your foundations will be at the same level as theirs. When you say 'shallow foundations', does that mean part of your house doesn't have a basement? Basements are effectively a deep foundation, and were often built for that purpose.

Putting drains next to their basement shouldn't divert any water to yours, in fact it should pull water away from yours. But I don't follow "placing drains down the sides of the party walls" - I'm thinking of the party wall as being between your basement and theirs, and the drains being on the walls at right angles to those, front and back?

Party Wall sounds so much more exciting than the banal suburban reality.

Party Wall sounds so much more exciting than the banal suburban reality

Indeed...

Thanks,
The builder has dug an inspection pit to check foundation level today so I'll catch up with the neighbors later about this.

Our houses are built on a hill so that the basement at the front of the house is subterranean, but the back opens into the garden (at floor level).

The proposal for the drains, is for them to run next to the party wall & directly parallel. The fall would be from the front of the house to the back, where they can then plumb into existing drains I assume.

Both our houses have original basements. The reference to the shallow foundation was due to the victorian construction - usually nowhere near what would be laid to modern standards

Would this not come under a party wall agreement?

Useful reference here :
https://www.gov.uk/government/publications/preventing-and-resolving-disputes-in-relation-to-party-walls/the-party-wall-etc-act-1996-explanatory-booklet#para_28

Neighbours are having the work done and should appoint party wall surveyor who can make a record of condition of your property, this protects both parties if there is an issue later on, if they are good neighbours they should do this (plus it is legal, read guide). Drains and foundations are practical things, need building standards too, but neighbours should want this to be right....unless just developing and selling on, hence law to help. Hope this helps.

Thanks, yes this does come under the party wall act. Neighbors doing everything by the book so far as I can see.

My question is more regarding the proposed works, and whether they could present an issue or not. I.e. "excavating, but not below the level of the BASE of the foundations"

Is that ok?

Party wall agreements aren't worth the paper they're written on, they don't stop someone messing things up.

However, if they have a decent builder, who has done this sort of thing before then it will be fine.

NB Victorian terrace owner here and have excavated to foundations here and on both sides and no one died....

99.9% it will just be fine.

The Party Wall etc act 1996 covers this.

In this instance you are considered the 'adjoining owner'. Your neighbour carrying out the works is considered the 'building owner'
They should appoint a surveyor to serve a notice to you under the relevant provisions of the act (from you describe it would be a section 3 notice describing elements of work covered under section 2 of the act. You will then have 3 choices,
1-agree to works - nothing further happens they crack on as they wish
2-Dissent to the works and agree for their surveyor to act as agreed surveyor to prepare an award covering the condition of your property, the type and nature of their work and any agreed limitations (methodology, timing etc) or
3-appoint your own surveyor who will agree an award the same as the above with their surveyor. Either way you will not pay penny, the building owner picks up all costs.

I would recommend option 2 as a minimum or ideally option 3. The nature of the works is very intrusive on your property and likelihood of damage high. An award will put a clear line in the sand on working methods and techniques as well as agree the condition of your property BEFORE any works start.

PS been a party wall surveyor for a long time!

The proposal for the drains, is for them to run next to the party wall & directly parallel. The fall would be from the front of the house to the back, where they can then plumb into existing drains I assume.

They are digging up the floor of their basement and laying drains under it, parallel to the party wall? In that case the excavation will almost certainly be below the foundation of the party wall, because the basement IS the foundation. So that excavation needs to be well supported.

Just realised I misunderstood your OP: I was assuming next door were digging down and tanking their basement on the outside. Drains parallel to party wall implies they are tanking the basement internally, with a drainage cavity and new inner skin wall. So what I said in my post above about their drains taking water away from yours may be wrong; depending how they're doing it, it could make your side of the party wall damper.

Lots of questions to be resolved it's definitely Party Wall Act territory. You need to ask them what their engineer is proposing to avoid undermining the wall, how they are going to avoid making your wall damper, where is the water table, etc.

Both our houses have original basements. The reference to the shallow foundation was due to the victorian construction – usually nowhere near what would be laid to modern standards

You mean the foundations of the basement at the back, because in effect it's not a basement at that point? In that case I agree, I don't expect they go much deeper than the floor.

the basement IS the foundation

Maybe I should explain what I mean there. The purposes of digging down for foundations include:

- get below any loosely compacted soil from previous use of the land

- get below the zone likely to be disturbed by tree roots

- start the walls from ground that has been pre-loaded by the weight of soil on top

A basement does all these, and additionally has the benefit that because the basement is full of space, weight is removed from the ground, so that the weight of the building isn't making subsidence any more likely. Since the ground at the bottom of the basement has been very well pre-loaded, there's no need to go any deeper. Some tall buildings in cities can have 5 levels of basement, essentially the basement is a hollow box foundation.

A modern basement will have a concrete slab floor and the walls built from that. A Victorian basement will probably just be a hole in the ground with the masonry laid on the bottom of the hole and some slabs laid as a floor.

Thanks for responses all, very very useful, much appreciated 👍

From your description, it sounds like they are installing a cavity membrane system (Delta), whereby any water penetrating the wall falls down behind the membrane and collects in perimeter channels. These are laid to falls to a sump chamber.

The perimeter channels usually need a chase forming in the floor slab (120mm wide x 75mm deep was the dims of the last one I oversaw a few weeks ago).

If this is the case, they may not actually be digging down & excavating through the slab & beneath the footings (probably corbelled brickwork).