Adding Storeys To An Existing Building

Adding floors on top of an existing building is one of the most efficient ways to create new space in Sydney, because the land, the footings and the lower structure are already paid for.

It is also one of the jobs where the engineering decides the yield. How many storeys you can add is rarely a planning number alone. It is the answer to a structural question: how much spare capacity does the existing building have, and what does it cost to find more?

Start with what was built

The first step is finding out what is actually there, which is often different from what was drawn.

Original structural drawings can sometimes be found in council archives or with the strata manager. Where they exist, they are a starting point, never the final answer, because buildings get altered over decades and drawings get lost or superseded. Where nothing exists at all, the process in [assessing an existing building with no drawings](/blog/assessing-an-existing-building-with-no-drawings) applies.

On site, we confirm the structure by measurement and testing. Cover meter scanning locates the reinforcement without breaking anything out. Concrete cores give the real compressive strength and show the condition of the steel. Exposing a footing or two confirms its size and depth against what the drawings claim.

Where the capacity runs out

Each part of the load path gets checked for the extra weight, from the new roof down to the ground.

Footings are the most common limit. They were sized for the original building, often with little margin, and extra load means extra settlement as well as extra bearing pressure. Underpinning or new piles inside an occupied building is possible, but it is slow and expensive.

Columns in the lower floors carry the accumulated load of every storey above them, so a column with plenty of capacity at the top can be fully used at the bottom.

Transfer structures, where they exist, are often already working hard, and they are difficult to strengthen without major disruption.

The roof slab that becomes a floor was designed for roof loads, which are lighter than floor loads. It may need strengthening to carry people, partitions and services, using the methods in [strengthening an existing slab for a new load](/blog/strengthening-an-existing-slab-for-a-new-load).

Weight is the whole game

Every kilogram added on top travels all the way to the footings. That makes the construction method of the new levels the single biggest decision on the project.

A conventional concrete floor is heavy. A steel frame with a lightweight composite or timber floor weighs a fraction of it, and mass timber floors can be lighter again. The trade-offs between heavy and light construction are set out in [lightweight construction versus masonry](/blog/lightweight-construction-vs-masonry). On a building near its footing limit, choosing a light system can be the difference between adding two levels and adding four.

Lighter floors carry their own checks, mainly acoustic separation and footfall vibration, but those can be solved with detailing. Footing capacity usually cannot be solved cheaply.

Wind and earthquake change too

A taller building catches more wind, and a heavier building attracts more earthquake load under AS 1170.4.

Both of those flow into the lateral system, the walls and cores that keep the building from swaying. The existing lateral system may have been designed to older standards, and the additional height and mass can push it past its capacity even when every column and footing is fine for vertical load. New shear walls or bracing inside an existing building are disruptive, so this check belongs at feasibility, not at detailed design.

Lifts, stairs and fire

Extra levels need vertical access. Lift shafts and stairs have to extend up through the new floors, and an existing building without a lift may need one to make the new floors usable at all, with the work described in [adding a lift to an existing building](/blog/adding-a-lift-to-an-existing-building).

Fire separation and fire resistance levels for the new structure are set with the fire engineer and the certifier. Depending on the scale of the work, the certifier may also require parts of the existing building to be upgraded, which is worth knowing before the feasibility is locked in.

Building over people

Most vertical extensions are built over an occupied building.

That means the existing roof has to stay watertight during demolition and construction, cranes and materials have to be managed over occupied space, and noise and vibration have to be controlled. Temporary works and sequencing are part of the structural design, not something left to the builder on the day.

How we approach it

We test the existing structure early, before the architect draws the new floors, and give the owner a clear answer on capacity and where it runs out.

That answer shapes the design: how many storeys, which construction system, and where strengthening is needed. Done in that order, a vertical extension is a good use of a building. Done the other way round, the architect designs four floors and the footings say two.

ACSES provides structural engineering, existing building assessment and strengthening design across Sydney. Talk to us about a building.

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