What is FSR — and how it decides how big you can extend

If there’s one planning number worth knowing before you extend, it’s your FSR — Floor Space Ratio. It’s the control that sets the maximum floor area you’re allowed to build on your land, and it’s written into your council’s Local Environmental Plan (LEP).

The simple version

FSR is a ratio of building floor area to land area. Your maximum Gross Floor Area (GFA) is:

Max GFA = Land area × FSR

So a 600m² block with an FSR of 0.5 has a maximum GFA of 300m² — that’s the total floor area allowed across all levels combined, not per floor. If your existing home is already 250m², you have roughly 50m² of floor area left to add before you hit the ceiling.

Why it matters for extensions

FSR is the reason two identical-looking homes can have very different potential. It tells you:

• Whether you have unused capacity to extend at all — and how much.
• Whether a second storey, a ground extension, or neither, fits within the limit.
• If you’re already near the ceiling, where the value really lies (often quality and layout, not more floor area).

It works alongside other controls — height limits (often around 8.5m for two storeys), setbacks, and site coverage — but FSR is usually the headline constraint on how much you can build.

How to find your FSR

Your FSR is mapped in your council’s LEP (Clause 4.4) and published on the NSW Planning Portal. Reading it correctly matters: it’s an authoritative numeric value, not a band or estimate. Our free Block Assessment reads your address against the official FSR and land data and shows you, in plain terms, whether your block still has room to grow.

This is general information, not advice for your block. Zoning, FSR and structure differ site to site — the only way to know what your land can do is to check it. Start with our free Block Assessment, and for a definitive answer, a Lot Strategy Brief.

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