Drainage Diagrams in Contracts

A residential property cannot be advertised for sale until a Contract of Sale has been prepared and the contract must contain a copy of the title documents, drainage diagram and a current Zoning Certificate (s 149) issued by the local council.

Please note that the drainage diagram does not come as part of the 149 Certificate but is a separate document included in a contract of sale.meeting audit plan business agent

A Drainage Diagram shows the layout of the property and location of the private house sewer lines on a property to where they connect to the authorities waste-water system and the location of the Council’s sewer mains, it is an official document lodged with your local Water Board.

The diagram can also be called a sewerage service diagram (SSD) or property sewerage diagram.


The Drainage Diagram will let you see where the original plumbing was and if it has been moved, or any other work that has been done.

Any adjustments to the diagram will be clearly stamped by the local Water Board,

Here are some typical stamps:

  • Work not finalised
  • Final inspection required

•  Work does not comply

Your Conveyancer will investigate what, if any, problems are associated with the stamps especially in relation to ‘Work does not comply’ as this signifies there could be a serious problem if a plumbing issue was raised by an inspector and has not been fixed.

The diagram must have a current date, if there is no date your Conveyancer will follow up to ascertain if renovations have been completed and not inspected for final approval.


A BOS letter is a ‘letter of Approval for Building over Sewer’ (BOS)

It is issued by the local Water authority and provides a description of where part of a house or other improvements have been constructed over the sewer.

It tells you if a building that has been built close to or over main sewer pipes has been approved by both the Water authority and the local council.

The BOS must be provided BEFORE Settlement and would be known as a prescribed document it needs to be attached to the Contract of Sale or the Purchaser can walk away from the Contract within 14 days after the date of exchange of Contracts.

Want more of this type of information?

Garth Brown


Garth Brown is the Founder and Director of Brown and Brown Conveyancers one of Sydney’s leading Conveyancing Firms. With 20 years experience as a practicing Conveyancer he has assisted hundreds of Clients to navigate through the maze of conveyancing issues when buying and selling property. Visit

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