Good to hear this has triggered review........re the basement costs, it depends upon how it's done. Either way you are doing 2 other separate garages that would be rolled into one. it involves negotiating levels, grading driveway at the higher end to get down so there could be some increased complexity.....but it's hard to make an accurate comment on numbers...... Catherine when you are doing something of this scale it's critical you are working with people/designers who are ALL OVER the costs in relation to design decisions and options. If they can't provide you with this advice and it's just design/plans/drawings that then get handed back to you to discuss potential costs with a builder, then I'd argue they're not up to the task. Before we do anything we robustly thrash out just numbers and project scope relationships (ie no detailed drawing!) and it deals effectively with the major issues and project alignments at the right time, and mitigates the nasty surprises or wasting resource pursuing options that are not viable. Either way, as I previously commented, as it's drawn currently it's a relatively expensive design that is arguably not delivering you the value/potential that you'd expect for an expensive design if you get what I'm saying. You will be paying a certain amount for a split, staggered or two level building (which is essentially two residences in one) and when you're getting into this type of construction ( basement or garages + driveway with a building of certain complexity & size) it will pretty much cost a similar ballpark whichever way you configure it.........BUT the outcome with be DRAMATICALLY different one way or the other, comparing designs of different levels of quality. Good design is not necessarily more expensive, rather it is better considered and gives you more for your money.

Hi oklouise, In the shorter term, I was going to live in the granny flat and rent out the main house to people I know or 2-3 international uni students. In the longer term, I would like to have it as a lowset living option for my mum who is in her late 70's. Somewhere, low maintenance, where she can keep her independence (as would I), but has me close by to provide her assistance where required. The original house has 3 bedrooms (the 3rd one is only 3.1 x 2.65m with no built in wardrobe. The back of the property backs onto a council park. The house is set back further from the front than all of the neighbouring properties (greater than 7 metres). The area under the current house would just be used for carparking and storage. The remaining length of my backyard from my back steps is 18.6 metres. I don't think it would necessary add value to my property, but rather provide me with an income as well as options for family or friends that need the accommodation. I hope to keep the house for the long term.

Make sure you insulate your new walls together with the walls of the existing house. Homes with all walls insulated properly will make the house twice as comfortable and can reduce the heating and cooling costs by 70% 4 Seasons Home Insulation

i would also prefer to have the granny flat face north but it really depends on the local climate so my suggestions includes the laundry within the flat in favour of a bug screened mudroom alfresco area between the two buildings and a smaller verandah but, adding the wide internal doors and arrangements better suited to potential disabled use and keeping guests out of the bedroom has resulted in an ens/wiw/dressing area without direct connection to bedroom but there's more storage, dressing and standing space and night time users of the bathroom won't disturb sleepers so much ... the study is big enough (with or without a separate hallway) to be a small bedroom with room for double desks and a single bed and the kitchen has space for wall oven, big fridge, small island, reach in pantry and lots of drawers but i'v left out the external laundry door in favour of a front door into the living area...total floor area of the granny flat (excluding alfresco and verandah) is 102SQM including brick veneer external walls