What do all these terms mean and what's the difference, if any? It's an important question and one that's too often overlooked and in some cases if you purchase a property with one of these descriptions, you may or may not accurately understand what you are buying. Let me do my best to explain.
What is a Dual Key:Starting with the most simply of the options a Dual Key is simply a property that has 2 separate areas operated by different keys. This was first made popular in CBD's such as Brisbane, Sydney and Melbourne, where 2 bed, 2 bath apartments were sold as dual-key apartments with a common key to the front door and then a second key for each occupant to their own portion of the dwelling. It can apply to a number of different type of dwellings but importantly, does not convey any indication of separate services (i.e. water, power, etc).
What is a Dual Occupancy (often shortened to Dual Occ.): stated correctly, this is two dwellings on a single lot. Generally it implies separate services and at least the potential to subdivide at some point in the future, albeit with perhaps some work to be done to achieve this.
Dual Occupancy is probably the most over-used term in property marketing and is often misused to represent Primary and Secondary Dwellings (which we'll get to next). So if you're looking at a property that is being described as a Dual Occupancy, make sure you understand what you are looking at and what you are purchasing.
One easy way to confirm it's actually a dual occupancy is to ask for a copy of the Development Approval or Planning Permit (depending what state you're in).
What is a Primary and Secondary Dwelling: A primary dwelling is a main standard dwelling on a property, i.e. usually a traditional 3-4 bedroom home. A secondary dwelling is the planning term for what is more commonly known as a granny flat. These can be attached or detached.
One of the more important considerations when looking at this type of property is that it will rarely be marketed as a Primary and Secondary Dwelling (which is what it is), more commonly they are called, Dual Occ's, Dual Key's or the somewhat more ambiguous, Dual Living. It doesn't really matter what it's 'called' as long as you understand what you are buying and the inherent limitations.
While Secondary Dwellings (Granny Flats) are permitted in most parts of Australia their use is restricted. Currently, in Victoria and most of Queensland (Ipswich City Council excepted) you cannot legally use a Secondary dwelling as an investment property to be rented out, the occupants must have some relationship with those of the primary dwelling.
Often the approval process for a Primary and Secondary dwelling is more simple and does not necessarily require a full Development Application or Planning Permit, however it also comes with constraints (apart from who can legally live in it).
Notably if your property is in NSW and has been approved as a Complying Development, then it can NEVER be subdivided. This is not up to your local councils discretion, it's a function of the Affordable Housing State Environmental Policy under which they are approved.
The Take Home Message?
Understand what you are buying, make sure the use you propose is legal in the the location you're buying it and if you're thinking of subdividing the property at some point, make sure you can.
Dual Occ's aren't 'better' than Primary and Secondary dwellings, they're just different and sometimes for you, the cost savings associated with one dwelling type and approval type may outweigh the flexibility down the track of taking a different route.