Checking foundations to add an additional storey to a West Oxfordshire property

SWJ Consulting was asked to visit a property on Curbridge Road, Witney where the owners were looking to complete a significant refurbishment including adding an extra storey to the house.

We also looked to design a flat roof infill between the existing cottage and garage with a ceiling light as well as ensuring a cantilever staircase could be incorporated into the existing structure to provide access to the new storey.

To achieve the upward extension the roof line needed to be lifted but first the client had to estaThe bungalow dated back to the mid-1900s, and we needed to assess whether the existing foundations would be suitable to support the weight of an additional floor.

We conducted a site visit and, after coordinating trial pit locations with the client, performed a back analysis to confirm the foundations’ suitability for an attic truss conversion.

The property had traditional trench foundations, and we needed to calculate the weight of the additional floor, along with the existing structure.
As we are local to Witney, based on our knowledge of area and site investigations we’ve been part of over the last 10 years, we surmised an allowable bearing capacity of the Cornbrash sub soils.

Our knowledge of the area is based on numerous local site investigations we have conducted (see map below).

This provides extensive knowledge of ground conditions and helps us identify potential variations, for example, caused by historic river movements.

On another scheme, we helped a developer save time and money by recognising that ground conditions would likely vary across their particular site. Targeting the investigation based on this knowledge, provided data to distinguish the soil differences between a historic riverbed and the surrounding area which allowed a reduced foundation size and depth in certain areas.

Foundation design is often approached with a worst-case mindset—where a single poor soil sample could dictate the strategy for an entire site. However, by leveraging our local expertise and recognising the potential for variable ground conditions, we were able to plan a more strategic investigation. This proactive approach allowed us to distinguish between different soil characteristics early on, ensuring that the developer had a set of foundation designs optimised for each plot rather than applying a blanket solution. The result? Significant cost savings and a more efficient, site-specific foundation strategy.

When we do not have specific knowledge of a site, reference can be made to the British Geological Survey, which provides geological data and bedrock information. Combined with trial pits and or further intrusive investigation techniques, through ourselves or our geotechnical partners, helps us assess soil conditions and their impact on foundation design.

The image below shows the superficial geology of West Oxfordshire, including Long Hanborough where an old river (thin green line) ran through the development.

This provides extensive knowledge of ground conditions and helps us identify potential variations, for example, caused by historic river movements.

West Oxfordshire features a mix of Oxford Clay and Cornbrash limestone (locally known as Limestone Cornbrash).

In some areas, old river terrace deposits—comprising gravel, flint, or clay—may be present, as these superficial layers follow the paths of ancient rivers.
At SWJ Consulting, we never assume a worst-case scenario. Instead, we conduct thorough ground condition investigations with our geotechnical partners, backed by our expert local knowledge and site investigation database.

If you are considering a significant extension or basement for your home then give us a call on 01993 225085 (Witney) or 02381 920656 (Southampton) or email mail@swjconsulting.co.uk for a chat or contact us through the website. If you consult with a structural engineer before an architect, we may be able to save you money on designs that are not feasible, and help guide your architect on what designs are achievable.