Plan everything & get quotes for all elements, electric, plumbing, insulation, sound proofing... strongly consider solar passive, energy rating, double glassing, water recycling, ongoing maintenance, organic gardens, shrubs, fruit trees, fences (style & cost). Uni Queensland has some young engineers developing Tiny houses plans & fabrication so Google them. Investing in an Owner builders license would be a good insightful investment. Know the costs right down to council fees before you commit.

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.

we've had several new slabs built over the last few years and each time the engineer inspected the grounds and took soil samples as well as designing specific thickness and extras for the slabs two of which involved extra underground concrete piers on sloping sites ..we've also had an engineer inspect and design new floors to enclose a room over two separate old concrete verandahs ..one of which involved removing the original concrete and starting new and the other suspended slab had a 10cm slab added to raise the height to match other floors (and assume that would have also included some waterproofing) ... we've also had engineers inspect and advise about structural changes for renovations and advise about cracks in an old double brick building and each time price was based on hours of work and i can't be specific but assume at least $1000 for basic inspection and taking photos (maybe an hour or two on site) plus they need a copy of any existing and proposed plans so a few hours for off site research and a written report with plans and recommendations for any new and/or remedial work and each job will have individual challenges

is the garage attached and part of the original house or a separate building or extension?...we need to see a site plan showing all buildings and a plan of the whole house showing room sizes, doors, garage doors, windows and location of existing plumbing.. ..what is the shape of the whole roof (Google earth view and some photos would help) and what are the roof, walls and floor (of the house and the garage) built out of?...is the garage at a different floor and/or ceiling level than the house, how will you use existing bedrooms and do you need a new carport/garage? BUT before you make too many plans you should check with local council to clarify what will be needed for the conversion because a garage is not usually built to be suitable for bedrooms or living areas and 10m x 4m is probably big enough for 2 beds and a bath or 2 beds and a small living area but not for 2 beds + bath + living area