Can I build a duplex or granny flat in Thornleigh?

The typical Thornleigh block

The middle block of the 25 we checked is 729 m² with 16.8 m of street frontage. The smallest was 436 m² and the largest 2,150 m². Zones in the sample: R2 (25).

What homes sell for in postcode 2120

Median sale prices for the whole of postcode 2120, January to March 2026. These cover every suburb sharing the postcode, not Thornleigh alone, and they are not a value for any one block.

• Houses: $2.01m (42 sales)
• Units and apartments: $1.37m (49 sales)

Source: NSW Government Rent and Sales Report (Department of Communities and Justice), sales tables by postcode, NSW Government. Updated each quarter.

A granny flat

20 of the 25 blocks we checked could take one. Hornsby LEP cl 5.4(9) — the greater of 60 m² or 33% of the house’s total floor area, parking excluded. Separately, the state Housing SEPP allows a granny flat of up to 60 m² through the ten-day complying-development route on a lot of 450 m² or more with a 12 m frontage, where the land is not flood, bushfire or heritage constrained.

Dual occupancy (duplex) rules in Thornleigh

14 of the 25 blocks we checked could carry two or more separate homes today.

Under Hornsby Local Environmental Plan 2013 a dual occupancy needs 700 m² and 16 m of frontage.

Of the 19 ordinary R2 blocks in our sample sitting outside the Low and Mid-Rise area — where that minimum is the only test that applies — 12 had the frontage and 15 had the land. 10 had both.

So in Thornleigh it is the frontage that does the stopping — the land is there on plenty of blocks, but they are narrow and deep. Width is the harder of the two to solve.

Low and Mid-Rise

6 of the 25 blocks sat inside the state Low and Mid-Rise area — within about an 800 m walk of a listed centre or station. Inside it the Housing SEPP overrides the LEP minimums: a dual occupancy on 450 m² with a 12 m frontage, terraces on 500 m² / 18 m, townhouses on 600 m² / 12 m and a manor house on 500 m² / 12 m, up to 9.5 m. That is what lifts the two-homes figure above; a block one street outside the line gets the LEP figure instead.

What else turned up

11 of 25 were on bushfire-prone land; 2 of 25 carried a heritage listing or conservation area. Any of those takes a block out of the fast-track routes and into a full DA, so it changes the answer more than the lot size does.

How we got these numbers

We took 25 real Thornleigh street addresses from the NSW geocoder and ran every one through the same engine this site runs for you, reading the planning controls live. Nothing here is modelled or estimated.

It is a sample, not a census. The addresses are a spread across what the geocoder returns, so treat these as "of the 25 blocks we checked" rather than a figure for every block in Thornleigh. Your own block is the only one that answers for itself — that is what the box on the right is for.