How big a granny flat fits on your Frankston block?
On most Frankston blocks the answer is 50 to 60 m², and 60 m² is the practical ceiling: Victoria’s small second dwelling provisions let you build up to that size on a lot over 300 m² without a planning permit. At $1,900 to $2,600 per square metre a 60 m² build is roughly $114,000 to $156,000 before site costs. What actually decides your number is not the size of the yard, it is garden area, setbacks and how you get a person and a concrete truck down the side.
Start with the three limits, in this order
Every sizing conversation we have in Frankston, Seaford, Carrum Downs and Langwarrin comes down to the same three constraints. Work through them in order, because the first one that bites is the one that sets your size.
- The 60 m² planning threshold. Up to 60 m² on a lot over 300 m², with no planning permit needed, provided your block is clear of the overlays that switch the exemption off. Above 60 m² you are into a planning permit, which is months and money. Details on our Victorian granny flat permit rules page.
- Mandatory garden area. On lots over 400 m² in the residential zones, a percentage of the site has to stay as garden: 25 per cent from 400 to 500 m², 30 per cent from 501 to 650, 35 per cent above 650. Driveways and strips under a metre wide do not count towards it, which is what catches people out.
- Setbacks and access. A single storey wall generally sits a metre off the side and rear boundary, you need a path wide enough to actually use, and you need to get materials in. A 900 mm side gate on a battle-axe block can cost more in handling than two extra bedrooms would.
What fits, by block size
Typical Frankston City lots, assuming a standard single storey house already covering 30 to 35 per cent of the site, one metre setbacks and a usable access path. Treat it as a starting point, not a promise: a wide driveway or an easement changes the answer.
| Block size | Garden area required | Typical yard left | Realistic granny flat | Indicative build |
|---|---|---|---|---|
| 300 to 400 m² | Not applicable under 400 m² | 60 to 110 m² | 35 to 45 m² studio or small 1 bed | $66,000 to $117,000 |
| 400 to 500 m² | 25 per cent | 90 to 150 m² | 45 to 55 m² 1 or 2 bed | $85,000 to $143,000 |
| 501 to 650 m² | 30 per cent | 140 to 210 m² | 55 to 60 m² 2 bed | $104,000 to $156,000 |
| 651 to 800 m² | 35 per cent | 190 to 260 m² | 60 m² 2 bed plus a real yard | $114,000 to $156,000 |
| Over 800 m² | 35 per cent | 250 m² and up | 60 m², or larger with a planning permit | $114,000 to $156,000 plus permit |
Build ranges are $1,900 to $2,600 per square metre, the same band used across our pricing guide. Site costs sit on top: see Frankston site costs for what those actually run to on sand versus the clay and slope in Frankston South.
Size it, then price it
Set your block size to see how much yard you have to play with, then set the granny flat size to price it. The estimate is the build, not the site works.
How to measure your own block in twenty minutes
- Get the plan of subdivision or the title diagram. It shows the real boundaries and, critically, any easement. You cannot build over a sewer easement without an agreement, and South East Water will want a build-over application if you are within a metre of the main.
- Tape the usable rectangle at the back: boundary to the house wall, and fence to fence. Take a metre off each boundary the building will sit near.
- Mark the access path. Allow 1.2 m to walk a wheelbarrow, more if a bathroom pod or wall frames have to come through rather than over.
- Photograph the meter box, the hot water unit, the sewer inspection point and any big tree. Those four things move more granny flats sideways than council ever does.
- Subtract the garden area percentage for your lot size from the whole site, not the backyard. This is the step people skip and it is the one that shrinks the plan.
If you would rather not do the arithmetic, send the address and we will pull the property report and tell you the number. It takes us about ten minutes and it is free.
A worked example: 650 m² in Carrum Downs
A typical 650 m² block in Carrum Downs with a 190 m² house and a 40 m² driveway. Garden area at 30 per cent means 195 m² has to stay garden, and the driveway does not count towards it. The backyard measures 14 m by 12 m.
- Usable rectangle after 1 m setbacks on the two boundaries the build sits near: 12 m by 11 m.
- Take out a 1.2 m access path and a clothesline you still want to reach: about 100 m² of workable area.
- A 60 m² footprint fits with 40 m² of yard left over, and the garden area test still passes because the lawn beside the house and the front garden count.
- Build at $1,900 to $2,600 per m²: $114,000 to $156,000.
- Sand footings in Carrum Downs, straightforward services, one crossover already there: site costs at the lower end of the range on our site costs page.
Same house on a 480 m² block two suburbs over and the answer changes to 45 m², because 25 per cent garden area plus the existing footprint leaves a smaller rectangle.
When smaller is the right answer
We will say this plainly, because it costs us work: not everyone should build the maximum. If the granny flat is for one parent who wants their own front door and a garden to sit in, a 45 m² one bedroom leaves the yard intact and saves around $28,000 to $39,000. If you are chasing rental yield, the two bedroom at 55 to 60 m² is the better buy because the rent difference outruns the build difference. And if you need three bedrooms, a granny flat is the wrong instrument entirely: at that point an extension or a knockdown rebuild is the honest advice, and we are not the right builder for it.
The other case where we say no: a battle-axe block with a 900 mm side and no rear lane. It can be built, but crane hire and hand-carting can add $15,000 to $30,000, and the money is better spent on the house you already have. See what happens on the day for how access shapes the build programme.
Sizing questions we get every week
How big can a granny flat be in Victoria without a planning permit?
How much backyard do I actually lose?
What does garden area mean for my block?
What size granny flat do most Frankston families build?
Does the size include the veranda or the eaves?
Is a bigger granny flat always better value per square metre?
Send us the address and we will tell you what fits
We pull the property report, check the overlays and easements, and come back with a maximum size and a build range within one business day.
Not sure whether your block clears the overlays?
Free on-site consultation across Frankston City and the Mornington Peninsula, fixed written quote inside 7 days.