We're in the process of building at the moment, wall framing is being fixed in place as we speak.
As a first time home builder it's a steep learning curve, and just when you think everything is sorted you'll think of something else, or a better way of having something, or you'll get a bunch of questions from the building company you'd never even considered.
Once you have a floor plan you are happy with, sit on it for a few weeks. It's amazing what improvements you come up with even when you initially thought it was exactly what you wanted. If you can spend a couple of weeks without seeing anything that can be improved, then you're probably OK to take it to the next step.
Everyone will have a different opinion on your chosen design, just remember that not everyone needs to love it - as long as you and your family does. So I won't give advice on the actual plan.
Be wary about doing too much stuff solely for future resale appeal (unless you know you will be selling within a few years). But this has to me tempered with not doing anything that would be extremely off-putting to any future buyer. It's a bit of a balancing act.
You will get a lot of advice (especially here on Geekzone) about things you should upgrade. Much of it will be along the lines of "if I was building I'd definitely...". Not all of this advice will be coming from people who have built, or who know how much these things will cost. Most of these suggestion will certainly improve your house, but they all come with a price tag. Most upgrades on their own don't cost much, but once added up might add $50,000 or more to your build price. When we started the process I thought we'd be able to do all these things, but we just haven't been able to.
Get a very clear understanding of what decisions you need to make, and at what stage of the process they need to be made by. And be prepared that there will be way more decisions to make than you expected.
For what it's worth here are some of the upgrades that we felt we could justify; either because we felt they should eventually pay for themselves, or they were just something we really wanted. If it's beneficial I can give you the prices we were quoted on some of these if you flick me a PM (we didn't get everything quoted, as we reached the limit of our budget and didn't even consider some of them):
- Ducted heat pump and heat recovery fresh air ventilation. 6 zones with electronic baffles and wireless temperature sensors in each zone.
- Excel Low E glass
- Laminated glass
- Thermally broken aluminium joinery
- R2.8 insulation in 90mm external walls (including garage) and between garage and house, R4.0 in 140mm external walls. Garage door was already insulated as standard.
- R6.0 insulation in ceiling (including garage)
- Higher efficiency gas califont for water heating
- Kitchen and laundry went a good amount over PC sums.
What we didn't upgrade:
- Change all external walls to 140mm to allow R4.0 around entire perimeter (we were approaching the maximum site coverage already, so didn't even get this priced as it would have pushed us over)
- Any thermal improvements to the RibRaft foundation, such as thermal edging.
- Additional membranes etc to make house more airtight.
- LVL framing
- Tiling or laminate flooring in kitchen,toilet,laundry, and bathroom (we are tiling the ensuite floor and ensuite shower, but all other hard floor locations are vinyl planking).
- Under tile heating in ensuite
- Garage carpet
- Door/window architraves
- Higher skirtings
- Higher stud (although we do have a higher ceiling in entrance, and a vaulted ceiling in living/dining)
- Overheight doors throughout
- Profiled doors