Somewhere on a stack of paperwork is a string like "Lot 5 on RP812345" - the reference a title search, a building application, or a conveyancer will eventually ask for. Most owners have never memorised it and cannot produce it on demand. A lot number identifies a parcel in a state's official land title and cadastral records - for example, Lot 5 on RP812345 in Queensland, or a lot and Deposited Plan (DP) reference in New South Wales. It is not the same as a street number, and that gap between the two is where most of the confusion starts.
Check a boundary without needing the lot number - free
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The lot number appears on your Certificate of Title, the original plan of subdivision or Deposited/Registered Plan for the property, and in some states on a rates notice or a council planning certificate. If you have any of those documents, the reference is printed on them directly - there is no need to search a government portal from scratch.

MapMyLot does not require a lot number at all. Search the street address on MapMyLot and the matching cadastral parcel resolves automatically, showing the boundary polygon, land area, and perimeter - useful when you want to confirm a boundary or lot size quickly and do not have a title document to hand.
A lot number is still required for formal transactions - a title search, a building or development application, or a conveyancing matter - because it is the legal identifier land registries use, not the street address. For those, retrieve it from the title or planning certificate rather than guessing from a map.
Note: cadastral boundary data is subject to positional variation by location and source dataset and is sourced from government records - see ICSM, the intergovernmental cadastre coordination body for background on the national cadastral program - for informational purposes - not a substitute for an official title search.
Five address lookups a day are free on MapMyLot, with no account required - enough to confirm a boundary and lot size for a handful of properties without a lot number. MapMyLot Pro, at A$19.99 a month, removes the daily limit and adds unlimited PDF boundary reports for anyone doing repeat lookups.
Formatting conventions vary by state:
Both identify the same kind of thing: the registered parcel reference behind a property's street address.
See how lot size is calculated, or check whether a fence sits on the boundary.
Check a boundary without needing the lot number - free
Check an address freeFive free address lookups a day, no account needed. Additional readouts depend on local data coverage.
MapMyLot currently covers:
Search your address directly to confirm. An address outside these areas will not return a boundary, and MapMyLot will say so rather than showing a guess.