For a house to be sustainable and energy efficient, in needs to be well insulated. It needs high performing windows. Thermal bridges and air-leakage need to be minimised. Ideally the house should have some thermal mass. And the house should have good solar access. But that is kind of the main thing. The actual roof form or the wall cladding/wall finishes are up to your personal taste.
My main advise is always to spend as much money as you can on windows and insulation. While you will most likely update your kitchen and or bathroom in 15-20 years, most of us will never ever touch the windows or the insulation again.
How much it will cost to build your energy efficient and sustainable home does depend on a lot of factors. If you are building a home on a budget size does matter. Simply put, the larger your home is the more it will cost. The more features you want the more it will cost. The bigger the house the more materials and labour is needed, the more money you have to spend.
If you do want to build an affordable home you kind of have to pick and choose certain features and finishes for your home, depending on how much money you have available. Not everything in your house can be a feature.
Also, you should be open when it comes to window and more so door types. The most expensive option are bi-fold doors and windows, not only does the system cost a lot. But a retractable fly screen for a big door can easily cost $3000 alone. The next step down is stacker sliding doors. Even more affordable are standard sliding doors, where some panels are fixed. But even than changing to a French-door can save you a few more thousand dollars.
One of the first steps should be to really think hard what it is that you really need. Think about clever use of space and multi-functional spaces. And you also need a designer/architect that can design a beautiful compact home that will feel bright, open, and spacious, without being huge. Avoiding and minimising corridors and walkways is a key. Avoiding build-in furniture and custom-made pieces where possible.
High performing windows do not need to cost an arm and a leg. We have several contacts for high performing double glazed upvc windows and even triple glazed windows, that come in cheaper than many standard low performing window systems. More than happy to pass on details.