Can I build a duplex or granny flat in Mount Colah?

The typical Mount Colah block

The middle block of the 25 we checked is 843 m² with 19.2 m of street frontage. The smallest was 477 m² and the largest 1,154 m². Zones in the sample: R2 (25).

What homes sell for in postcode 2079

Median sale prices for the whole of postcode 2079, January to March 2026. These cover every suburb sharing the postcode, not Mount Colah alone, and they are not a value for any one block.

• Houses: $1.77m (11 to 30 sales, a small sample)
• Units and apartments: not published (10 or fewer sales this quarter)

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

A granny flat

22 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 Mount Colah

18 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 25 ordinary R2 blocks in our sample sitting outside the Low and Mid-Rise area — where that minimum is the only test that applies — 21 had the frontage and 22 had the land. 18 had both.

Both tests bite here in roughly equal measure, so a block needs to be unusual on two counts at once. That is why the number clearing them is small.

Low and Mid-Rise

None of the 25 blocks we checked fell inside the state Low and Mid-Rise area, so in this part of Mount Colah the LEP minimums are what count. The boundary is a walking distance from a listed centre, so it can cut through a suburb — the checker reads which side of it your own address sits on.

What else turned up

13 of 25 were on bushfire-prone land. 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 Mount Colah 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 Mount Colah. Your own block is the only one that answers for itself — that is what the box on the right is for.