Offset foundations: For when you're tight to the boundary

The typical low rise domestic foundation detail to the left might be familiar to you. It's taken from the ​Building Construction Handb​ook 10th Edition, written by Roy Chudley & Roger Greeno.

First published in 1988 it's been one of the most popular reference books for anyone attending college and university construction courses. 

It lays out quite clearly how foundations work where the load is applied centrally onto a minimum 150mm thick concrete strip foundation.

A 600mm wide foundation gives a 150mm projection either side of a 300mm wide wall (equal to the depth of concrete) and this allows the load to be distributed at 45 degrees across the entire width of the foundation into the supporting strata below.

But what can you do when you can't achieve the 600mm spread because you can't trespass across a boundary or if there is already another foundation next door?

In certain situations it may not be possible for the builder to construct a foundation with a typical projection as that shown above. In these circumstances an offset (or eccentric) foundation, which is an engineered type of foundation, could be considered (subject to design). The image below provides a typical example of how an offset foundation might be constructed. 

The offset foundation

Care should be exercised when constructing foundations that are non-typical.

Even for some small and lightly loaded buildings, it’s not always possible to give definitive advice on the type of features that should be used, as other factors might need to be considered. Consequently, when constructing an engineered foundation, advice should always be obtained from a qualified and competent structural or civil engineer.

An engineer will be able to analyse the load on the foundation, the thickness and depth of the concrete, the mass of the concrete, and the type and position of any reinforcement in conjunction with the strength of the soil that the foundation will be placed upon.   

If the site conditions are not suitable for either a traditional or offset foundation, you will need a structural engineer to design another foundation type such as a raft or piled foundation.

Read more about working with foundations

• Underpinning existing foundations
• How to get it right: Building a wall on foundations
• Foundations on clay soil
• Trees and foundations (foundation depth calculator)
• When and how to cut stepped foundations on a sloping site

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.

_{This article was updated on August 2024}