A guide to extension costs - part 4, ground work

Part 4 of this series about the cost of an extension we are looking at how much it costs for the groundwork.

The groundwork includes the foundations, drainage, electrical supplies, water supplies, and the base pad. The base pad includes excavation, type 1, sand, waterproof membrane, insulation and concrete.

The groundwork and base pad is one of the main areas of development that makes it really difficult to quote for.

When setting budgets it is suggested that homeowners allow for a potential 25% overspend, I am sure you have seen on the TV programmes that few people come in on budget due to unforseen complications and/or the owners deciding to upgrade in the build process.

When estimating the cost, once a building is at ground level, is more easy to determine.

With the ground works, despite test pits and analysis of the area we are not certain of what lies beneath the surface until we start to dig.

Providing a rough price for preparing the groundwork and base with any degree of accuracy without a site survey is not possible.

We need to determine the soil type, how deep foundations need to be dug, the drainage required, what services may lurk beneath the project area. Access is also important, will it need to be hand dug or can a digger get access. How will waste be removed, will the waste be contaminated and how much will it cost to dispose of it. And, of course, size matters.