Thinking about adding a basement space to my flat - needs some advice please

Shaun1000
Shaun1000 Posts: 11
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Hi 
Firstly thanks for reading. 

To explain my property - its a detached former police station dating back to 1850. its a bit of a labyrinth due to the amount of extensions it has had over the years and Its worth saying for the purpose of this thread that the property backs onto a river, and between the river and the property is a 2 meter wide concrete lintel.  

i have the ground floor flat and there is one upstairs flatm, between us we own 100% of the freehold on a 50/50 split. The building is not listed but is in an area of some sort of historical importance - i forget the correct word.

Underneath my floorboards is about 4 feet of downward space and then the ground. i can see lots of stuff from yesteryear some of which doesn't look supportive, some does. 

What i want to know is if its possible to build myself a basement by lowring the ground further and making another room. What would be the process to do something like this. I,e to get permission first? structural engineer first? or would you approach your upstairs neighbours first - how do i get their blessing officially? basically where do i start!

If you have done something similar in your home, please can we chat directly? any advise comments or critique welcome, as i am a bit out of my depth with this one (forgive pun), but keen to know process/ ball park costs / do's and don't for this type of project. 


THANK YOU
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  • stuart45
    stuart45 Posts: 3,824
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    Were you thinking about DIYing this?
  • Hi mate, not really, i would get stuck in where possible, I think the best i could do is the excavation part as the only way i see this being done is one shovel full at a time? As for the rest of the structural stuff, tanking and so i would need experienced help
  • stuart45
    stuart45 Posts: 3,824
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    There are firms that specialize in doing basements, as this isn't standard construction work. Usually much more expensive than a normal extension, so is normally only done in areas like London with high land values.
    An initial survey would be for things like water table height, ground conditions etc. to see if the job is possible.
  • Emmia
    Emmia Posts: 2,961
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    edited 16 November 2023 at 9:05AM
    Given the age of the building it's possible the foundations themselves aren't very deep, i.e. they might not be much below what you can see.

    Basement excavations are very popular in London, but expensive and disruptive on neighbours (that's those on either side, as well as above). You may therefore find some resistance to your plans - particularly if people think that your excavations will damage their property.

    Check a trade reckons £2,500 - £4,000 per square metre... So costs could quickly mount up for a relatively small space.
  • hmm ok thanks for that - i do think the foundations are probably very shallow. If that is the case, would it mean that the job isn't viable or i'd need to repin the house in order to move forward?

    I also don't want to rack up a massive cost but i do live in London and the only way to extend (for me anyway) is down.. so i see this havinbg a positive affect on my property price as well as adding an additional living space. 

    Fortunately i dont have anyone to the sides of me and the next nearest building is a good 60 or so feet away. The only people that might find it a nuisance is my upstairs neighbours who i own 50% of the freehold with.  On the other hand the upstairs flat has the benefit of the large roof or loft space - which is shared but i dont really get the benefit of - i see this the same as the ground beneath my flat.

    Do you know if planning permission is also required for basement work?


  • FreeBear
    FreeBear Posts: 14,262
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    Emmia said: Check a trade reckons £2,500 - £4,000 per square metre... So costs could quickly mount up for a relatively small space.
    £2500 to £4000 per square metre is about right for a conventional above ground extension. Digging out a basement, you would be looking at considerably more. The technical challenges are much higher as is the labour costs. Get it wrong, and the consequences could be catastrophic - https://www.standard.co.uk/news/london/finchley-family-left-homeless-after-bodged-building-work-causes-house-to-collapse-a3193121.html


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  • Nobbie1967
    Nobbie1967 Posts: 1,447
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    I would be very wary of starting a basement excavation close to a river because of the potential for cost escalation once the digging starts. Also I would be concerned about insurance cover for a basement near a river. I wonder whether you might be better off trying to rationalise the current hodge podge of extensions into some more usable space?
  • Thanks @FreeBear thats extremely sad to read but i take that on board - obviously i don't want my house/flat to collapse. 

    @Nobbie1967 - understand your point, perhaps not so much a 'hodge podge'  but i was more pointing out that areas of the building are 'newer' than obviously the original part. For instance the bedroom area is filled with a concrete balaste rather than it being a floating floor. 

    I think more thought needs to be applied - perhaps a structural engineer would be a good place to start to view it and get some hard facts... 

    Thank you everybody. 
  • Section62
    Section62 Posts: 7,505
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    Shaun1000 said:
    Hi mate, not really, i would get stuck in where possible, I think the best i could do is the excavation part as the only way i see this being done is one shovel full at a time? As for the rest of the structural stuff, tanking and so i would need experienced help
    It is very unlikely the foundations will go down far enough to be below the level you'll need to excavate to, and with a nearby river it is likely there will be problems keeping water (and ground) out of the hole.  It is likely the optimium solution will involve minipiling, but that won't be easy in a labyrinth.

    With retrofit basements the excavation part is in fact highly technical - the walls and surrounding ground will need constant monitoring for movement, and the rate and location of each part of the excavation needs to be planned and managed to keep everything under control.

    This is bad enough when it is only your own home, but with someone else's home sitting on top of yours you have the added complication of making sure you don't damage their property (or kill them) with your own work.  Frankly if I was your upstairs neighbour I would only be happy if a very experienced contractor was doing the work... and if they have generous and up-to-date third-party liability insurance.

    This isn't a DIY project in any real sense - you'd need to allow for the cost of employing a fairly expensive contractor, and to be handing your home over to them for several months (i.e. you'll need to move out) while they do the digging and structural work.

    If that hasn't put you off, the first step would be to get an architect and structural engineer on board to start working out which (if any) of the internal walls can be removed to make the project more feasible.
  • Emmia
    Emmia Posts: 2,961
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    An Architect may also be able to suggest an alternative to remove/reduce the hodgepodge of existing rooms and give you a more workable home, without the basement excavation.
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