HI Kath,
If I have read your question correctly, you are asking whether you can knock down the existing garage conversion and rebuild it, and in addition convert your attic into additional habitable space?
If the situation allows, and you have permitted development rights on your property you may be able to do both without planning permission under permitted development. However there are a number of of conditions that you must fulfill, an Architectural Professional will be able to assist with that.
Redeveloping a bungalow is entirely possible, however It wouldn't be advisable to submit a planning application (Outline or Full) on a property you do not own as the above answer suggests.
The current owners would be notified through the planning process, it will cost you money to have a set of application drawings drawn up and you will be adding value to a property that you do not own. A planning approval can add significant value to a property and it would be very likely the vendor would up the selling price.
The best way to establish if what you want to do is viable, would be to discuss it with the local Planning Department and obtain some Pre Application advice for a nominal fee without submitting a planning application.
I hope that helps,
Kind Regards,
Rob Whittle - Hertfordshire Design