Sub-Soil Survey: Getting it Right First Time

[Pemu]

Morning everyone,

I have made an offer on a piece of land subject to satisfactory sub-soil survey which I need to get done. 

I am keen on exploring the idea of adding a basement to the build and need to get a survey done that gives me sufficient information. 
Naturally, I want to save as much money as possible as well. I also want to have all the information I need to them get quotes, work out costs, etc., to then decide whether or not I want to build a basement. 

I've read posts where people have had to get a second survey and I want to avoid such costs. 

This is all new territory for me so any advice and information will be very helpful, including recommendations for good companies who serve the North London area.

Questions that come to mind:

To what depth do the samples need to be taken from?

How many holes need to be dug and where?
What details do I need to have included in the report?

I'm sure there's more. 

 

The plot is at the end of a terrace so there's a pavement and then a road next to it. My goal is to include a basement under the main footprint of the house which is about 11 metres long and 6 metres wide. There will be a further 2.5 metres between that and the border of the property/pavement where I will try to get permission for a single story side extension separately. 

 

I'm attaching a picture of the outline of the property. 

 

Any other general advice for this stage will be helpful. 

 

Thanks

 

 

[Ferdinand]

I think all of those are properly answered by your pro adviser as each site will be different,  though you may be able to get some idea if you can get information on soil types etc, or by digging your own hole first. With luck it will be bedrock at 2 feet.

 

My one comment would be to look to eg Milton Keynes for your engineer, rather than London.

 

Ferdinand

 

Edited June 8, 2019 by Ferdinand

[Russell griffiths]

I carried out my own soil survey, with a mini digger and lots of common sense, 

maybe you could talk to basement construction companies and have them do a trial hole to see if they find anything scary. 

 

Have you got a rough ball park figure for a basement, in that location, my thoughts would be the costs will be the highest you will find anywhere in the country. You will also need to be careful of not overspending. 

 

This is coming from a former north west London resident, who built the most expensive end of terrace house in the street. 

[Conor]

The drawing shows the proposed dwelling being attached to the existing neighbouring dwelling?

 

If you are considering a basement, proximity to existing buildings may be an issue, depending on soil conditions, your neighbour's attitude and planning policy regarding substructures.

 

In regards to  geotechnical investigations, you'll be wanting to drill bore holes to 50% beyond the bottom of the proposed basement. E.g 4.5m for single level basement. Geological Survey online maps will give you an idea of local ground conditions. They also have borehole logs, if luck you might find some that were done nearby.

 

Basement construction costs start at approx double the going rate for ground level construction. E.g. if local rate is £2k/m², then budget £4k/m² for basement. Adverse ground conditions could potentially double or triple that number.

 

We're doing a basement as well... But have advantage of a sloping site so will be a lot easier.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[ToughButterCup]

8 hours ago, Pemu said:

(...) 

Questions that come to mind:

To what depth do the samples need to be taken from?

How many holes need to be dug and where?
What details do I need to have included in the report?

(...) 

 

Take proper professional advice from three specialist survey companies. 

 

Ask for references.

 

Check their professional qualifications.

 

Ask them to list membership of a relevant professional body

 

Check the relevant standards policed by that organisation 

 

Check the currency of their Professional Indemnity Insurance .(ask to see their certificate of insurance) 

 

Network : ask other people about the company. When I did that I was surprised at the directness of some of the answers. 

Once that's done you have enough information to make an informed choice. And reduced your exposure to unnecessary risk. 

Good luck! 

 

[Mr Punter]

You will need to establish the soil type and strength, neighbouring foundation depth (as the existing house will need underpinning) and ground water levels (as a basement that is below the water table is considerably more problematic).  You may need to include contamination assessment as well.  The cost of the soil survey is fairly low in the great scheme of things.  You should also establish the exact location, depth, material and diameter for the sewer.

 

You will need a Party Wall agreement or award with the neighbour.

 

You will need a design from you architect / engineer for the upper floors as that will impact the basement design, especially the ground floor type, proposed basement waterproofing and basement to ground floor junction.

 

You will then be able to use the soil report and architect drawings to get a price from some basement companies for the whole job so that they are the main contractor for the basement phase. I would want them to include excavation, underpinning, temporary works, drainage, welfare facilities, structural engineering, site security and all insurances so that they hand over a completed and signed off basement.  Will cost plenty.

 

[mvincentd]

 Talk to a potential structural engineer and ask what they would want the soil survey to cover, because they will design the basement based on that survey  so you need to give them what they want.

 

[Pemu]

Thanks everyone so far. 

Some clarifications and some more questions:

 

Yes, the existing landlord is splitting his garden and has also agreed to any party wall requirements, etc. 

The Thames Water report shows a 150mm waste pipe running through the middle of where the basement would be so I'm investigating costs and implications on that. I made a separate thread about that: 

 

Are there any other checks that you can think of which I should get done (e.g., for other underground services)?

 

Is this the correct website for doing some background research and finding data from other boreholes?

https://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/viewer.html

 

I haven't come across anyone else locally who has a basement so can't think of where to get a ballpark figure. 

 

@Ferdinand did you have anyone particular in mind from Milton Keynes or was there a reason you were suggesting looking there? 

 

 

Also, can anyone recommend any companies and companies for getting the survey done whom they have used themselves, heard good things about or would just be good yardstick for prices, etc.?

 

 

Edited June 8, 2019 by Pemu

[Russell griffiths]

Is a basement in that location really worth the cost??

have you got a ballpark figure of what you think it will cost. 

[Tony C]

1 hour ago, Russell griffiths said:

Is a basement in that location really worth the cost??

have you got a ballpark figure of what you think it will cost. 

+1 on this one, It is not worth it unless your house is in premium location in London?

Friend of mine just completed below terraced house basement in London.

Soil Survey was £3.5k including couple of Trial pit to investing current foundation, 2 bore holes 6m depth and lab test.

Finished basement 35-40m2 costed was £100k+VAT (incl. one bathroom)

You will be paying extra 20% on top on VAT as it is not new build, which is a big factor!

[Pemu]

At the moment, I'm at the stage of investigating whether the cost will be worthwhile. Based on what I am hearing so far, it seems more likely that it won't be, but I still want to explore it thoroughly before I make that decision and that includes getting a soil survey done. 

 

Regarding the VAT, to my knowledge, it will be a new build. At the moment, it's someone's garden with a garage on it. Is there anything that disqualifies it from VAT exemption?

 

 

[Bitpipe]

9 hours ago, mvincentd said:

 Talk to a potential structural engineer and ask what they would want the soil survey to cover, because they will design the basement based on that survey  so you need to give them what they want.

 

 

This is what I did. 

 

The SE are ultimately the customer of the soil survey as they'll use it for loading calcs, slab design (i.e. whether piles are needed). They wrote the spec and tendered it out.

 

We had a mix of 10m probes and 6m cores plus gas and water monitoring for 3 months after, plus contamination testing (for LA condition and muck away classification).

 

We had to have a second survey due to some errors in the first and general uncertainty about the ground conditions (suspected made ground based on historical maps).

 

It's still way cheaper than starting to dig a hole and then seeing what you find and if it's going to be too expensive to build it, you can drop the idea.

 

Our basement is one of the best parts of the house tbh, Just over 100m2 split into two pairs of interconnecting rooms - kids use one half for TV, music etc and the other half is a gym and craft room/reading room etc. Lovely bright space, always at 20^oc. Cost was £1000/m2 before fit out but we had a very accessible detached site and had demolished the original so nothing in the way.

 

 

[Ferdinand]

I was saying look away from London not towards it, in general - but not so far out that they do not understand the extra hoopla.

[Tony C]

Sorry, I was not reading your post property , you are building in London! 

I am also building in London, waiting the tender cost come back from several contractors for the house which has about 25m2 basement.

We have completed the Site investigation with 10m borehole and lab test.

The Structural engineer originally got some quotes from Site Investigation companies, but they were quite expensive. I looked some help on Buildhub forum and got good recommendation for the SI company, which was much cheaper and did good job.

Please PM me I can send you the SI report example etc.

 

[Tony C]

22 minutes ago, Bitpipe said:

Our basement is one of the best parts of the house tbh, Just over 100m2 split into two pairs of interconnecting rooms - kids use one half for TV, music etc and the other half is a gym and craft room/reading room etc. Lovely bright space, always at 20^oc. Cost was £1000/m2 before fit out but we had a very accessible detached site and had demolished the original so nothing in the way. 

I am also building a basement (25m2), and will be using UFH. Did you go for UFH? I am wondering if the spacing of the pipes needs to be wider as the basement will be more insulated? Although I will be installing a different thermostat for the basement...

[Bitpipe]

1 hour ago, Tony C said:

I am also building a basement (25m2), and will be using UFH. Did you go for UFH? I am wondering if the spacing of the pipes needs to be wider as the basement will be more insulated? Although I will be installing a different thermostat for the basement...

 

I did not put any heating in the basement but did build the 300mm slab off 300mm of EPS200 (50mm sand blinding, 150mm compacted type 1).

 

Basement walls are insulated outside with 200mm EPS70, clad in 3mm corex to protect when backfilling.

 

End result is that the basement is 20^oc year round  - no heating needed, solar gain from the lightwells, ambient heat from TVs, a fridge, and the plant room all seem to give it the heat it needs.

 

MVHR supply to all rooms also.