Granny Flat Plumbing in Brisbane: What It Costs and What Council Wants
Secondary dwellings are one of the fastest-growing build types in Brisbane and Logan. The build itself is straightforward — it's the plumbing connection, council approval and drainage design that catch owners out. Here's what actually happens.
Connecting to the Existing Sewer
A granny flat has to discharge into the same sewer connection point as the main house. That means a licensed plumber sets a new drainage line from the flat back to an existing inspection opening or a new junction on the property drain.
The run needs correct fall (typically 1:60 for 100 mm sewer), correct venting, and inspection openings at each change of direction. Getting fall wrong is the most common reason a new granny flat gets recurring blockages.
Water Supply and Metering
Most Brisbane secondary dwellings are supplied off the existing water meter with a branch line. If you intend to rent the flat separately and bill water usage, you can install a sub-meter — cheaper than a second Urban Utilities connection but it needs to be installed by a licensed plumber to be readable for billing.
Hot Water for the Flat
A dedicated unit is almost always better than extending off the main house. A small continuous flow gas unit or a 125–160 L electric tank suits a one-bedroom flat. Sharing the main house's system means long dead-legs, slow hot water delivery and wasted water.
Stormwater and Drainage
Roof water from the new dwelling has to be lawfully discharged — usually into the existing stormwater system or to a legal point of discharge at the kerb. Brisbane's clay soils don't absorb quickly, so soakwells are rarely the answer on their own.
Approvals and Certificates
Drainage work on a secondary dwelling is notifiable plumbing work. Your plumber lodges the required forms with council, and you receive certification on completion. Keep it — it's requested at sale.
Realistic Costs
Budget roughly $6,000–$14,000 for the full plumbing scope of a one-bedroom granny flat: sewer connection, water supply, internal rough-in and fit-off, hot water unit and stormwater. Long runs, rock, or a difficult sewer connection point push it higher.
Frequently Asked Questions
Do I need a separate water meter?
Not legally in most cases, but a sub-meter is worth it if you're renting the flat out and want to on-charge water usage fairly.
Can I connect the granny flat to the house's hot water system?
You can, but we usually advise against it. A dedicated unit gives faster hot water, less waste and simpler billing.
Is granny flat plumbing notifiable work?
Yes — the drainage work is notifiable and must be lodged with council by a licensed plumber.