Stormwater drainage from a site can be one of the greatest obstacles in granny flat approval. This is due to the fact that compliance with council’s stormwater policies must be adhered to. This is the only part of the Complying Development process that reverts back to council’s local control plans.
Property Falls Towards The Street (higher than street)
When a property falls towards the street (is higher than the street), it is usually quite simple to connect stormwater. Connection to a downpipe at the rear of the house or directly to the street by a gravity fed line is usually easy.
Property Falls Away From The Street (lower than street)
When a property falls away from the street (is lower than the street), additional costs and approval time is required due to consultants, further processes and additional construction.
The best and easiest solution for this type of property is drainage via an easement available to the property.
SOLUTION 1: DRAINAGE VIA EASEMENT AVAILABLE ON THE PROPERTY
A drainage easement is a public (or private) pipe in the ground for the purpose of controlling stormwater. Although homeowners cannot build over easements, they are a blessing for stormwater drainage when a property falls away from the street.
If an easement is not available, then alternate options must be investigated. The following may not be compliant with council policies in all areas:
SOLUTION 2: DRAINAGE VIA A CHARGED LINE
This is the second preferred method of drainage. A charged line is where water is pushed uphill from the roof of the granny flat to the street. The gravitational force of rainwater is what makes this work as the water is pushed through the pipe which then exits through the kerb outlet onto the street. When rain stops, water does remain in the pipes and eventually dries up. This is the reason for using PVC downpipes to ensure no leaks.
This is a similar concept to the ‘S’ trap beneath your kitchen sink that catches waste. There is always water in the pipes but when you turn on the tap, water is pushed through by the force of the water.
SOLUTION 3: DRAINAGE VIA AN ABSORPTION PITT
An absorption pitt is usually our last resort. This is where a large hole is dug out approx. 6m long x 1m wide. Geotech fabric is installed. PVC domes area placed in the trench. Geotech fabric is then wrapped around the dome. Stormwater pipes are connected into the dome.
Bluemetal is spread over and around the dome and then 300mm of topsoil is installed over the top.
An absorption pitt is similar to an underground rainwater tank.
The collected water from the roof of the granny flat will eventually absorb into the ground (diperse and disappear).
An absorption pitt will only work when the soil is suitable. In certain areas, a geotechnical testing and report is required to determine the type of soil. Heavy clay and rock areas are not suitable for absorption pitts.
An absorption pitt must be located on the lower part of the land and setback at least 3m from fence lines and 2-3 m from any structure making it a little challenging when positioning a granny flat.
In the old days, these were called rubble pitts. We’ve certainly come a long way.
Sometimes, there are existing ‘Rubble Pitts’ as the stormwater solution for the existing primary dwelling. These are usually located in the back yard. The existing rubble pitt must be identified and removed as it can create a very wet back yard. More importantly, a granny flat cannot be built over a rubble pitt as the rising moisture will create future problems.
When none of the above options are available, then, it may be possible to request an easement through your neighbour’s property. This is a significantly expensive and lengthy process.
DRAINAGE VIA AN EASEMENT THROUGH NEIGHBOURS YARD
Sometimes there are no other options available to homeowners but to request a private easement through a neighbour’s property. Fair market value must be offered to a neighbour for the portion of land required to create the easement. The easement will create restrictions on their land. There are legal, survey, registration and construction costs applicable in addition to the compensation given to your neighbour.