Longer and more expensive - generally - because it usually means building the block to maximise it's potential. For example at R15 it might be suitable for a single dwelling, at R40 it becomes a triplex. So they would generally allow the 'upgrade' if you propose to build a triplex. They might not allow it if you propose to put a house in the backyard and only make it a duplex.
This is by no means concrete - some councils will stipulate on the website what you need to do but it's best to call the Planning Dept and get further information.