A second storey addition, and whether your footings can take it

A second storey addition, and whether your footings can take it

The question I get asked is always about the upstairs. How many bedrooms, where the stairs go, whether the roof line works. The question that decides the project is about the downstairs, and specifically about what is under it.

A single storey house was designed to carry a single storey. Add a level and you have roughly doubled the vertical load on every wall and every footing beneath it. Sometimes that is fine. Sometimes it is the reason the project changes shape.

What has to be established before design starts

Four things, and none of them can be assumed from the age of the house.

The footing itself. Width, depth, whether it is reinforced, and what it is founded on. That means digging test pits at two or three locations and looking, because a 1960s strip footing and a 1990s slab and beam are different propositions and both exist all over Sydney.

The soil. A site classification under AS 2870 tells you how reactive the clay is and how much seasonal movement the footing is riding. A site that was marginal for one storey becomes a design problem for two.

The walls. Single skin brick, cavity brick and timber frame all behave differently under added load. Cavity brickwork carries load in the inner leaf, and if the wall ties have corroded, the wall is not the composite element the original builder intended.

The floor and roof structure. The existing ceiling joists were never sized as floor joists. They are almost always replaced, which is a cost people plan for late.

The three findings that change the project

Undersized footings. The fix is widening or underpinning, which is achievable and adds cost and programme. It is better found in a test pit than in a crack six months after handover.

Poor founding material. If the existing footing is bearing on fill rather than natural ground, the added load has nowhere sensible to go. Piles down to competent material become the answer, and that changes the budget significantly.

Wall capacity. Where the existing masonry cannot take the new load, the alternative is often a lightweight upper level, timber or steel framed with light cladding rather than brick veneer. That can bring the loads back inside what the existing structure carries and save the footing work entirely. It is worth testing that option early rather than treating it as a compromise at the end.

Why councils ask for a structural adequacy report

The consent authority is not asking whether you can build a second storey. It is asking whether the existing building can carry it.

That report has to state what was inspected, what was found, what the structure can take, and what work is required to make the proposal viable. It has to be based on physical investigation rather than on drawings, because the drawings for a house built in 1968 either do not exist or do not match what was actually built.

We cover this under structural engineering, and it usually runs alongside a geotechnical investigation so the soil and the structure are assessed against each other rather than in isolation.

Timing that saves money

Commission the investigation before the architect finalises the layout, not after.

The reason is simple. If the footings can only take a lightweight upper level, that decision affects the plan, the roof, the stairs and the cladding. Finding it after documentation means redrawing everything. Finding it first means the design is built around what the house can actually carry, and the project runs without a structural surprise in the middle of it.