Adding a First Floor Extension Over an Existing Ground Floor Extension

Extending upward — adding first floor accommodation over an existing ground floor extension — is a popular way to maximise space without expanding the building footprint. But it is not as straightforward as building a new extension from scratch, because the existing structure was designed and built to carry the load of a single storey only.

Before PorthouseDean can design the structural additions, the adequacy of the existing foundations must be investigated and confirmed.

Step 1: Collect Existing Information

Start by gathering whatever information exists about the original ground floor extension:

– Original planning drawings and Building Regulations drawings — these may show foundation depths, widths, and types
– Building Control completion certificate — this confirms whether the original was built under inspection
– Any signs of differential movement: cracks in the walls, sticking doors, uneven floors, gaps at the ceiling

If the extension was built without Building Regulations approval, treat the foundation as unknown until investigated. If drawings exist but are old, treat the stated depths as indicative only and verify by investigation.

Step 2: Trial Pits

Trial pits are the standard investigation method for residential extension foundations. Typically 1–3 pits are dug adjacent to the existing foundations (or in positions that allow the foundation to be partly exposed) to confirm:

– Foundation type (strip, trench fill, raft, pad)
– Foundation depth below adjacent ground level
– Foundation width and bearing area
– Foundation condition (cracking, deterioration, inadequate width)
– Ground profile — what soil types exist and at what depths
– Groundwater — is the foundation dry, damp, or waterlogged?

This information is what the structural engineer uses to check whether the existing foundation can carry the additional load — and, if not, what underpinning or replacement is required.

Step 3: Engage Building Control Early

Notify Building Control as early as possible. They will want to know that foundation adequacy has been assessed. An early Pre-Application Discussion with the BCO can save time by clarifying what investigation evidence they require before they will accept the design submission.

Clay vs Non-Clay Ground

Ground type matters as much as foundation depth.

In clay soils, foundations can lose capacity due to seasonal shrinkage and swelling. Adding load to a foundation in shrinkable clay requires a check that the total bearing pressure remains within the allowable, and that differential movement between the new and existing structure will not cause cracking.

In non-clay soils (sand, gravel, chalk, rock), the main check is adequate depth and bearing area. Settlement is typically more predictable.

The Main Risk: Differential Movement

The biggest structural risk when adding to an existing extension is differential movement — where the new and existing parts of the structure settle by different amounts. Even small differences (a few millimetres) can cause visible cracking at the junction.

A lightweight first floor (timber frame rather than masonry, for example) adds less load to the existing foundation and reduces this risk. Discuss the construction form with your architect — the structural engineer can sometimes achieve the same outcome with a lighter superstructure where the foundation is marginal.

What PorthouseDean Designs

Once the foundation adequacy is confirmed (or underpinning design is complete), PorthouseDean provides:

– New floor and roof structure for the first floor addition
– Structural walls and columns for the new storey where required
– Any steelwork needed to span over the existing ground floor or to transfer loads to the foundations

Get started with your extension structural design — most drawings issued within 10 working days of instruction.