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.

why do you need the ensuite and what do you love about this property? based only on the floorplan i don't believe there's enough space to have an ensuite although, if this is a good house in a great area for a good price the best option would be to add an extension, subject to the usual costs and restrictions but, the ensuite or extension will sacrifice space from the existinhg house and even the example shown outside the house will only work if the side fence is at least 2m away btw your plans and measurements are deceptive because the bedroom next to the bathroom shown as 3.5m x 3.5m is really only about 3.5m x2.7m ... the attached copy of your plan shows some options for adding the smallest possible ensuite (shown as the rounded oblong at 2.5m x 1m) .. an extension allowing enough space to add a master bedroom with ensuite or repurpose existing rooms and add new living rooms with better connection to the garden could be an option but an extension is not cost effective just for a tiny ensuite

Assuming north is what you're indicating it's really poor planning putting a garage hard to the boundary on the north side and the majority of bedrooms on the south. Are you sure this is correct as typically north is up the page as drawn. If so the whole thing needs to be flipped/mirrored. Also there's arguably a significant amount of passage space in the house plan for what it is, not to mention a lot of unusable external space to the sides of the house. An alternative and more efficient design/floor plan would utilise the available space better for you and give you more flexibility with what you're trying to achieve....You're possibly pushing the envelope for what you're wanting on a single level for this particular proportion block of land. There would be a 2 storey option, (say with Master Bedroom on upper) which would achieve a double garage and all the other requirements, but appreciate it's always about practical cost limits. 6m to the back fence from the alfresco is not huge for a family home but depends upon your lifestyle. I'd be considering reducing the building footprint and really the only way to do that for a house this size is to consider a 2 storey format

Thank you for your comment Malissa, this one was a cracker of a garden that dictated terms to some degree with the lay of the land. By this approach it allowed all the elements to be in harmony. Kind regards Chris Slaughter - Lead Designer www.scenicbluedesign.com.au