It's always heartening to hear stories of people achieving successful and great value outcomes. My comment above was however not for standardised design or DIY scenarios, rather responding to an urban scenario with split level. The work we do is by nature customised and we deal with this sort of thing regularly & yes it (usually) involves a lot of money..as a flagfall and the sky's the limit. It's not however necessarily about bells and whistles and being fancy for the sake of it either. Rather it's about quality and long term value. You can spend a lot of money that lasts a long time which ends up being a more cost effective option than spending half as much but not lasting nearly as long. It's all relative and it depends of course also on land value, what is being proposed and the capacity for a proposed project to be financed. It's always a balance across a number of factors. We can rarely pin down a specific number either. It depends on specification and the design detailing. DIY construction is a completely different kettle of fish to a contracted build, as is a more simple, standardised design & construction/ specification to a more complex and customised multi/split level scenario with intricate/unique detailing...as is also rural scenarios to tighter inner urban situations. It really is a case of how long is a piece of string and it will range dramatically depending on how you are going about it. I had a client once who similarly was a DIY Owner builder with a trade background who did most of the work themselves. They similarly achieved the construction for a dollar amount far less that what it would have otherwise cost in a commercial contract. The design was also for mirror imaged connected townhouses so there was an economy of scale there as well, as well as being a compact and efficient design with certain details standardised to make the construction more efficient and materials relatively minimised with essentially 2 houses fitting within the size of a standard house, so they got 2 for the price of 1 essentially (good result!). BUT when you factor the practical hours "donated" or performed on the project for "free", including every hour of labour, construction management etc, supply and handling etc and apply a reasonable dollar value to it, as well as GST and builder's margin (or a construction management %) on top of the total, it's pretty much a resulting cost in the ballpark of what I was indicating.

As a ballpark starting figure use around $1800 - 2000 per square metre. This just a ballpark figure to use as as starting point and can easily vary either side. So for $150k for that size dwelling is only around $600 per square metre...... Contact a local designer (the good ones understand budgets and costings) and start from there. Material and product choices will vary the cost to construct greatly so keep that in mind. Building costs are always increasing not going down.

Almost impossible to estimate until some demolition has been done. Even then what is the final design and quality of fit out? Certainly get an architect on board with a project such as this, even more so if you've built before. While a building designer or a good draftsman can do the design, plans and documents for you (and they can do them to the highest standards by the way), the extra services that an architect provides will be required.

Yes, this is very much a 'how long is a piece of string' question! And the answer is probably very much a 'you get what you pay for' answer! We have just built a house and paid in the vicinity 0f $1800/m2 - the builder we used has 3 lots of 'specs' for houses - basically the lower budget, the middle, and the higher end. For our given floor plan they could quote on each of the specs and we went with the middle one. This involve things like the quality of the fittings/appliances etc. eg. with the low spec we would have got a basic kitchen with formica benchtops, the middle spec included soft-close drawers and hinges with stone benchtops etc and it was the same for all the fittings throughout the house - toilets/vanities/laundry fit out/quality of carpet/type of concrete for drive etc. A word of warning - we had a friend who was very happy because he built a house for just over $1000/m2 - but you could tell! It looked all fine when it was brand new but after a few months you could tell the poor quality of some fittings. And the carpets were already looking worn! so - you get what you pay for.