Can I build a duplex or granny flat in Castlecrag?

The typical Castlecrag block

The middle block of the 25 we checked is 728 m² with 16 m of street frontage. The smallest was 209 m² and the largest 2,394 m². Zones in the sample: C4 (18), R2 (6), E1 (1).

What homes sell for in postcode 2068

Median sale prices for the whole of postcode 2068, January to March 2026. These cover every suburb sharing the postcode, not Castlecrag alone, and they are not a value for any one block.

• Houses: $3.60m (33 sales)
• Units and apartments: $1.43m (36 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

24 of the 25 blocks we checked could take one. Willoughby LEP cl 5.4(9) — the greater of 60 m² or 10% 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 Castlecrag

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

Under Willoughby Local Environmental Plan 2012 a dual occupancy needs 700 m² — this LEP sets no frontage minimum, so Council's DCP governs width.

Only 6 of the 25 blocks we checked were ordinary R2 land outside the Low and Mid-Rise area, which is too few to read anything into — most of Castlecrag in our sample was zoned or mapped some other way.

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 Castlecrag 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

9 of 25 were on bushfire-prone land; 10 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 Castlecrag 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 Castlecrag. Your own block is the only one that answers for itself — that is what the box on the right is for.