Piled Foundations to Facilitate Retroactive Basement Digout

[Pemu]

Good morning everyone,

Hypothetical scenario I am exploring which very well could become a reality and I was hoping for your thoughts on an idea I had. I admit I have no idea if this is even possible to begin with let alone worth the money or hassle. 

 

The New build is in London. In the long run, we definitely want a basement but don't currently have the finances to do the full build with basement, etc. Need to get the shell complete and move in ASAP though as paying for alternative accommodation atm. 

What if we were to get all the relevant soil surveys done, have all the plans drawn up and spend extra on piling to make the foundations sufficient to support the house and not need underpinning later. Also, have the ground floor installed as if there is a basement beneath it.  A few years later, or when finances allow, we dig out the basement from outside without disturbing the rest of the house. Here's a YouTube series of someone digging a basement from outside the home without disturbing it: https://www.youtube.com/watch?v=W1LuFYIrUGw&list=PL51J3HlkH-iKNwCCk8krxfiNvLf9cEfyz&index=18&t=417s. If an entry point was planned in advance, we could do the same. 

 

Here are some pros and cons I can think off, if it were possible:

Cons:

1. The total cost would be more as the eventual excavation would take longer and be more tedious. 
2. The piling could/would cost more than digging the basement first and then using concrete strip foundations and reinforced retaining walls
3. If you dig basement first, you can make a continuous retaining wall with waterproof concrete and then potentially not need a drainage system. If you retroactively do the basement, does that mean you can't make such a wall and would unavoidably need the drainage? 
4. Doing the basement later would mean the walls would be thicker in total (piles+walls) making the internal area smaller.
5. If there are unforeseen issues in the basement build, much harder and more expensive to remedy.

Pros: 

1. Cuts the time until moving in to the house, hence saving money on alternative accommodation (which would offset extra cost)
2. Once moved in to the house, it should be much easier to oversee work, improve quality assurance and do more DIY (again hopefully offsetting extra costs)
3. Much less likely for the project to stop mid-way due to funds running out. 
4. Once all is said and done, more money has been spent on the actual fabric of the house.
5. Piled foundations and a basement drainage system, although more costly, provide more peace of mind and longevity?
6. Allows time to save up or arrange alternative funding (loan, etc.) for the basement. 

 

Will appreciate comments and thoughts on the above in terms of feasibility, costs, more pros and cons, etc. 

Edited November 8, 2019 by Pemu

[Conor]

I can see where you are coming from, but I don't think it would be viable in reality.

 

A much better option is to build a basic basement "shell". I.e. a concrete box that is just required water proofing E..g concrete floor slab, ICF walls, external tanking) and build the habitable spaces above. In future, you can fit out the basement and convert it to habitable space. That's basically what we are doing. We'll apply for planning. And BC in the future once we k ow what we want the basement to be. For now, it will be classed as storage and plant space. 

[Pemu]

Thank you @Conor for your reply. At your convenience, could you kindly elaborate on why you are sceptical about viability? I'm not saying you're wrong but am curious to know what challenges and obstacles you see. 

Regarding how you are going about things, what steps are you taking to ensure that in the future if you apply for planning you're not asked to redo any work you have already done? Is keeping pictures, etc., sufficient or do you have to get any particular inspections done, etc?

[Mr Punter]

If you are planning on contiguous piling to form the basement walls you may as well dig the middle out and concrete the floor.  Digging and muckaway is fairly cheap.  You need to consider daylighting, ventilation, drainage, insulation and waterproofing.

 

Waterproof concrete is very difficult and expensive to get right.  If you went with an internal drained cavity it is much less risk and you can do it later.

 

 

[Pemu]

Yes, so that is the idea I had in mind:

Contiguous piling for the walls but also forming the foundations for the house above (is that possible?)

Likewise, internal drained cavity was what I was thinking as there would be less to go wrong or at least easier to troubleshoot if it did?

As for daylight, ventilation, insulation, etc., what I had in mind was to get the designs done for the whole basement as I would eventually like it but then only do the essential structural parts than can't be done retroactively without too much cost. 

The basic philosophy is this:

If the whole basement would cost, for argument's sake, £100k to complete if done now, I would instead spend £20-40k now to get the bear minimum done and come back later (even in 10 years perhaps) and do the rest then as I can do without that space for now. Even if it then ends up costing a total of £120k (e.g., 30k now and 90k later), I would perhaps still consider that worthwhile due to the other advantages mentioned, e.g., being able to move in earlier, save up in between, stagger the cost, avoid spending all money on basement and then get stuck finishing ground and 1st floor, etc. 

 

 

[pdf27]

So far as I can see, the only difference in up-front cost is that you're using piling contractors rather than a groundworks guy with a digger to make the hole for the foundations/basement wall, you aren't paying for someone to dig the soil out of the centre while nothing is there (pretty cheap as access is the best it will ever be) and you aren't paying to lay a floor in the basement. It's worth costing out properly, but unless you have issues with deep excavations (e.g. close to a boundary) my suspicion is that a basement will be cheaper than piled foundations of the same depth. The extra work is all stuff that is really cheap to do now but expensive later, while piling (particularly if it has to be fully continuous) has a reputation for being expensive.