Thornleigh 2120 · Hornsby Shire
Thornleigh Builders: Homes for Three Generations
Grandparents, parents, a 24-year-old. One block in Thornleigh.
Thornleigh's big family blocks, the station and the schools keep people here for decades. Increasingly that means three generations under one roof, or at least on one lot. Designed well, everyone gets their own front door and nobody feels like a lodger.
Short answer
What are the options for multigenerational living in Thornleigh?
Three main ones: a detached secondary dwelling of up to 60m², a flat attached to the house, or a new home designed with an in-law wing. Thornleigh's 600 to 1,000m² blocks fit most of them. Bushfire-mapped lots go to Hornsby Council as a DA, usually 11 to 15 weeks. A mid-spec rebuild with an integrated second dwelling costs about $2,200 to $2,650 per m².
Written and reviewed by Oliver Alameri
Licensed builder, NSW Contractor Licence 487805C · Founder, Buildana
Fact-checked
Attached, detached, or built into a new home
There are three ways families here usually do it. A detached granny flat in the backyard gives the most separation, which suits a parent who values independence or an adult child who keeps late hours.
An attached secondary dwelling shares a wall with the main house but has its own entry, kitchen and bathroom. It's easier to help an elderly parent when they're a door away, and it can use less of the yard.
On a knockdown rebuild, the whole thing can be designed from scratch: a main home with a self-contained wing, sometimes on its own level with separate access from the side. On a sloping Thornleigh block, the lower level often works perfectly for this.
- Detached granny flat: most privacy, uses more yard
- Attached secondary dwelling: close by, separate front door
- Self-contained wing in a new home: designed in from day one
The trick is making it feel like two homes, not one house with a lodger.
What makes it work day to day
Separate entries matter more than people expect. So does sound. A shared wall between a teenager's music and a grandparent's bedroom needs proper acoustic insulation and ideally a buffer like a robe or a bathroom between them.
Each dwelling should have its own laundry, even a small one, and ideally its own outdoor space. Parking is often the sticking point on a busy street. Plan a spot for the second household's car before council asks.
For an older parent, the same principles as any accessible home apply: step-free entry, a walk-in shower, wider doors and reinforcement behind bathroom walls for grab rails later.
Planning for three generations?
Send me your Thornleigh address and who you're planning for. I'll check bushfire mapping, trees and slope, and sketch the options that suit your family.
0476 300 300Rules in the Hornsby Shire
The state's secondary dwelling rules let the extra home be attached or freestanding, capped at 60 square metres for a certifier's approval. Selling it off on its own title? Not allowed.
Bushfire is the big local factor. Much of Hornsby Shire is bushfire prone, and complying development isn't available for new dwellings on land rated BAL-40 or flame zone. On those lots, a DA with Hornsby Council is the path, usually 11 to 15 weeks.
Hornsby also protects its tree canopy, and many Thornleigh blocks sit on sandstone. Both affect where a second dwelling can sit and what the footings cost.
Costs and choosing the right option
A self-contained dwelling needs its own kitchen and bathroom, and those two rooms eat a big share of the budget. Rawlinsons has Sydney kitchens with plumbing and power at $19,500 to $22,000, bathrooms at $23,500 to $26,500. Size, slope and any rock excavation make up the rest.
On a full rebuild, a mid-spec home with an integrated second dwelling costs roughly $2,200 to $2,650 a square metre, straight Rawlinsons Sydney rates. We'll set that beside the cost of adding a granny flat to your existing house. Real numbers, side by side.
Questions to answer before designing
Six conversations to have as a family first.
- 01
Who will live in the second dwelling now, and who might in ten years?
- 02
How much independence does each household want?
- 03
Attached, detached or part of a new home?
- 04
Where does the second household park?
- 05
Is the lot mapped as bushfire prone, and at what rating?
- 06
Are there trees or rock that limit where it can go?
Multigenerational living in Thornleigh
Can a granny flat be attached to the main house?
Yes. Under the Housing SEPP, a secondary dwelling can be attached or detached, as long as it's self-contained and within the size limits for the approval path.
Is a backyard flat possible on bushfire-prone land?
Often, yes, but construction must meet the BAL rating, and complying development isn't available on BAL-40 or flame zone land. Those lots go through a DA with Hornsby Council.
Can I sell the secondary dwelling later?
Not on its own. NSW law keeps the flat on the same title as the main house, so it adds to the value of the whole property instead.
Is a rebuild with an in-law wing better than adding a granny flat?
It depends on your existing house and budget. If the house needs major work anyway, a rebuild with an integrated wing can make more sense. We'll price both.
Sources and references
Costs are from the Rawlinsons handbook with the local cost index applied; rules and timeframes are checked against the sources below.
- Hornsby Shire Council
- Rawlinsons Australian Construction Handbook, Rawlhouse Publishing
- Environmental Planning and Assessment Act 1979 (NSW)
- State Environmental Planning Policy (Exempt and Complying Development Codes) 2008
- State Environmental Planning Policy (Housing) 2021
- Building on bush fire prone land, NSW RFS
Everyone under one roof. Everyone with their own door.
A Thornleigh home designed for three generations, with the privacy and practicality to make it last.