Adding a granny flat or extra bedrooms on an unsewered block

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A lot of Lockyer Valley acreage isn't on town sewer, so a granny flat, a second home for family, or a couple of extra bedrooms raises an obvious question: can the new building use the system that's already there?

Sometimes it can. But you can't assume it, and you can't just run a new pipe to the old tank.

Why the existing system might not be enough

An on-site system was designed for a certain amount of wastewater, based on the house it was approved for. More people living on the property means more wastewater. Logan City Council lists having more people living in the home as one of the things that increases the load on a system (Logan City Council (opens in a new tab)).

The tank isn't the only limit. The land application area (where treated effluent goes into the soil) also has to cope with the extra volume, and the new building can't sit on top of it.

What gets assessed

Expect your plumber or designer to look at:

• the type, age, size and condition of the existing system
• how many bedrooms and people the property will have once the work is done
• the soil and the size and condition of the current land application area
• where the new building will go, and whether it encroaches on the disposal area or its setbacks
• whether there's room for a bigger or relocated disposal area if needed

Toowoomba Regional Council, the Lockyer Valley's western neighbour, says installing or altering an on-site sewerage facility needs a plumbing approval first, normally backed by a site and soil evaluation report, an on-site sewerage facility design and a drainage design (Toowoomba Regional Council (opens in a new tab)). City of Moreton Bay likewise requires approval before any alteration (City of Moreton Bay (opens in a new tab)).

The possible outcomes

1. The existing system can take it, possibly with minor changes, under an approved design.
2. The disposal area needs to grow or move, while the tank or plant stays.
3. The system needs upgrading or replacing. This might mean a larger plant, or replacing an old septic tank with a treatment plant. Any treatment plant installed must hold a current Queensland treatment plant approval (Business Queensland (opens in a new tab)).

Which one applies depends on the property. That's why a site assessment comes before a price.

Approvals in the Lockyer Valley

On-site sewerage work in the Lockyer Valley is permit work assessed by Lockyer Valley Regional Council. Applications go in on the state's Form 1 permit work application for plumbing, drainage and on-site sewerage work (council forms and fees (opens in a new tab)). Your plumber or designer normally prepares and lodges it.

The granny flat or extension itself may also need planning and building approvals. Those are separate from the plumbing approval. Ask council's planning and building teams what applies to your lot before you commit to a design.

Do the wastewater first

Get the wastewater assessed while the building plans can still change. Finding out late that the new building sits on the only suitable disposal area is an expensive problem. It's also worth checking the lot in council's free Flood Information Portal early (council (opens in a new tab)), because flood-prone ground affects where a disposal area can go.

Questions to ask

1. Can the existing system take the extra load, and what's that based on?
2. Do I need a new site and soil evaluation?
3. If an upgrade is needed, what are the options, and what do they mean for running costs and servicing?
4. Will the new building affect the current disposal area or its setbacks?
5. Who prepares and lodges the plumbing application, and what does that cost?

Related: New septic systems, Treatment plants, Replacing or upgrading a septic system.