Establishing Volume Change Potential of Soil for My Extension

1 September 2021
by D G

Hi all,

I'm looking to find out the most cost effective was of establishing the Volume Change Potential of the soil where our new extension will be built.

Using the NHBC Depth Calculator App, I need to find out the Modified Plasticity index to establish whether or not heave protection and a block and beam floor with a void is required as opposed to a standard sub base floor with screed.

Apologies if any of my terminology is out!

Can anyone provide any advice please?

Thanks in advance.

One Answer

  1. Chris says:

    Hi DG,

    Your local building control department may have some generic information about typical soil makeups in your area (based on past projects they’ve been involved with).

    However, the only way to establish things with a reasonable degree of reliability is to commission an expert to carry out a soil investigation and report (which would include an Atterberg Limits test for plasticity). Prices can vary widely depending on the extent of the survey, but typically start at around £1,000-£2,000 (assuming samples can be taken by hand).

    Bear in mind that even with testing, you can still sometimes be taken by surprise when you start digging in earnest. Hence why it’s vital to have a decent contingency budget for the foundation stages.

    We have a detailed guide to soil surveys here:

    Hope this helps!

    Chris (Editor, Build It)

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