Not only does the kid have a toy car in the box but also a black rocket ship!!!

It does not matter who builds the house (the article refers to a developer and that is what the person who builds speculatively is). If they do this for a living, then they pay tax on the profit, if you do it as a one off, then you may be paying CGT. If you are doing it to live in as your ppor, there is generally no cgt.

Spec home – house built for speculative purpose ie develop & quick sale.

I'd agree with Terry, the vendor has signed the DA, the DA relates to a specific site and cannot be separated  from that site. As you have done the hard work, it is your risk that if you can't settle you don't get any benefit (unless you have a claw-back clause

What you have interpreted is basically correct. If a new house (of the same quality & size as the existing house on the block of land) would cost $250k to build, the old house is about 25 years old, so would have 25 years of depreciation at 2.5% (about $156k),

So if the above is correct, then if the land is worth $150k, the existing (old) house about $93k – totalling $240k approx. Finding a run down house (for demolition & rebuild) at anything less than $240k can stack up. However you should research how much a new house in the area would sell for (it should be at least $150+$250+$93+gross profit) ie more than $500k (break even).

I know that this is heresy but if reasonable capital gains have been made on one or two of the properties, a tactic which could be used it to (dare I say it) sell one of these properties in order to pay down debt and get the interest beast under control. The thing to be aware of is timing, in order to qualify for a 50% discount on CGT you must have held the property for 12 months.

The other thing to be aware of is that variable interest rates are generally lower than fixed rates even in a rising market so there may not be a point to fixing rates if the banks have factored in the interest rate rises already.

Look at the purchase price of the old house, estimate the size, age & level of current maintenance/condition. Get your Rawlinsons out, work out what a similar house of the same size would cost (including landscaping etc), deduct x years of depreciation @2.5% and that would be close to the value of the land (roughly). Add back the cost to build the new house.

The only cash flow in a ng property is out of your pocket, as Terry points out you will have to weigh up all options & scenarios

Land size, land size, land size. That is the main constraint. Many councils willnot allow subdivision below x m2 so often you will not have the option of creating separate titles for each block. However, if you are clued up, you may be able to achieve a SEPP5 (or equiv in Victoria) allowing you to create 3 or more over 55's units on a block which would only give rise to a single house, eg 18m frontage with 35 m deep. However you need to be able to tick all of the other boxes to get the approval (the upside is you can get 3 units as opposed to 2 however they may take longer to sell).

As above, seek specialist advice. In the first instance you will need to convince your parent even before going to the solicitor. Unfortunately, wills can be changed, even whilst on your deathbed, so you will need to have the right protections put in place well before this eventuality. It also presupposes that your parent will be gifting their assets to you rather than using the assets to support their own lifestyle (rather than selling them and living the high life).