This is a complicated question. You could find that you don’t need to underpin at all as the current dead (self-weight) load and imposed (wind, snow and contents) load is not significantly increased by the proposed works. A structural engineer will be able to tell you whether it is required or not.

Underpinning can be a very dangerous operation if not carried out in a proper manner. My strong recommendation is that you take specialist advice before doing any works on site, including investigation. You should also consult a Planning Supervisor as you will probably need to comply with the Construction Design and Management Regulations (CDM).

If you need to underpin, the design depends on ground conditions and existing and proposed loadings, spans etc. There will almost definitely be various options, so cost is likely to be an important factor and most are carried out by specialist engineers and contractors, with public liability and professional indemnity insurance. Some can only be done by qualified people as they are very specialist, sometimes patented methods, needing training. Their involvement isn’t necessarily absolutely required, but you should check with your insurers that they will provide cover if you carry out the work yourself, both during construction and when the building is complete. The underpinning works will require a Building Regulations application if the building is a dwelling. From your description of the building as existing, you will need to make an application anyway.

Factors that need deciding will be depth of dig (trial holes recommended on at least 2 opposite walls), grade of base concrete, number of bases (these will depend on size and condition of building), what sequence the bases are excavated and poured, how long they must be allowed to go off before the next bases are excavated, how the bases are connected and crucially the method and material used for the dry-packing (this is making good the space between the top of the new concrete and the underside – known as the soffit – of the existing wall. This last is in itself a process needing very good workmanship, all else could be done well but if the last part isn’t, it can lead to very serious consequences. It can be done by people without experience, but I would strongly advise they seek professional advice. The soffit needs careful cleaning and preparation and the dry-pack needs very careful installation).

In summary, I would recommend the following :

1. Consult a local competent person such as a structural engineer. I would expect underpinning contractors to give you some free time on site or at least talk about it on the phone, perhaps with the aid of photos.

2. Dig a couple of trial holes on opposite walls as agreed with the professional. These should be large enough to dig down and inspect but small enough so they do not jeopardise the building or any person’s safety. They should show the depth of the soffit of the existing wall and if not into natural ground (which may need a specialist to determine), the depth natural ground appears. This may well vary over the site. You should avoid undermining the wall at this stage.

3. All of the factors I talk about above lead me to recommend you engage a structural engineer to design the underpinning. Tell them you would like to do the works yourself if that is the case and ask them to go through each stage with you in detail.

4. If you do carry out the works yourself, make sure Building Control inspects at the appropriate times. They will almost definitely want to see the first base(s) and will be looking at ground conditions, size of base(s), connections to future bases and how well the existing wall soffit has been prepared prior to dry-pack.

Good luck!

Jon Holleley, managing director at Surveyor Link Ltd