Pricing a building job with unknown ground beneath it

3 days ago
3 min read

Updated: 2 days ago

Introduction

Everything above ground on an extension can be priced with reasonable confidence. The blockwork, the roof, the glazing, the finishes — all quantifiable from a drawing. What sits below is not. Depth to a suitable bearing stratum, made ground, a high water table, tree roots, an old well, a drain nobody knew about, or clay requiring a deeper dig than assumed.

Foundations are therefore the element on which builders most often lose money, and the losses are large because groundworks are expensive and the discovery happens when the site is already open and committed.

The answer is to quote the ground separately and honestly rather than absorbing the risk invisibly.

1. Pricing a building job with unknown ground means quoting the ground separately

Do not bury it in a single figure.

Fix the superstructure and provisionalise the substructure

Everything above damp course firm; foundations quoted on a stated assumed depth with a rate per additional depth. Show the assumed depth prominently.

State the assumption explicitly

Priced on a trench fill foundation to a stated depth in firm ground. If it differs, the price changes by a stated rate. Put it on the first page of the quotation.

2. Reduce the unknown before quoting

Some of this is discoverable cheaply.

Dig a trial hole

A few hundred pounds and half a day converts guesswork into information on many sites. Offer it as a paid preliminary. Most clients agree once the reason is explained.

Ask about the site's history

Previous structures, filled ground, mining, trees removed, drainage runs. Old maps are worth a look too. Neighbours and the homeowner frequently know. Ask before you price anything.

Look at the neighbours' extensions

Anybody nearby who has built recently knows what was found. That is free intelligence. Knock on the door and ask.

3. Name the specific risks with figures

Vague exclusions do not protect anybody.

List what would change the price

Depth beyond the assumption, made ground, obstructions, root protection, a drain requiring diversion or a build-over agreement. Six named risks is enough.

Give a rate or a figure for each

The client can then understand the range rather than discovering it. Most clients accept a range and reject a surprise. Give the worst case as well as the likely one.

4. Manage the discovery properly

The moment the ground opens is the moment the relationship is tested.

Stop, photograph and price before continuing

Not after. A client shown a photograph of made ground at two metres understands immediately. Take the photograph with something for scale.

Get the variation agreed in writing

However obvious the necessity. Verbal agreement in a muddy trench is remembered differently later. A photographed signature is enough.

5. Do not absorb groundworks overruns

This is where small builders fail.

Charge for what the ground required

It is not your fault and it is real cost. Absorbing it once establishes an expectation for the whole job. Price it and explain it.

Keep the client informed of the running total

A shared, updated figure. Nobody disputes a total they have watched accumulate week by week. Update it every Friday.

Conclusion

Everything above damp course can be priced firmly; the ground cannot, so quote it separately with a stated assumed depth and a rate per additional metre rather than absorbing the risk invisibly in one figure.

Reduce the unknown first — dig a trial hole as a paid preliminary, ask about the site's history, and find out what the neighbours found when they built. Then name the specific risks with a rate or figure against each so the client understands the range in advance. When the ground opens, stop, photograph, price and get the variation agreed in writing before continuing, and keep a shared running total so nothing arrives as a surprise at the end.