The most important aspect of a hillside section is the slope. The steeper the slope, the more difficult it will be to build on and the more expensive. Building a concrete slab on the ground is cheaper below appr. 7 degrees slope. Above that, using a timber floor is cheaper.
cost comparison of a 8.7x17.4 meter slab.
A north facing would be prefered as you can build a house with good solar gain. Second best would be West facing. The afternoon sun is much warmer compared to the morning sun so you can still get good solar gain, but you will need good shading as in the summer your house might get overheated. Third best option would be East facing. Morning sun will be nice, but you will lose the sun early in the day. The worst option would be a South facing block esp. when the land is steep. A low sloped South facing lot may be OK, as you can still get some solar gains for most of the year. Another factor will be the views. The best sites have the best views towards the sun.
If you build on a slope you usually end up with a multi story house. Most people prefer to live on the top story because of better views. So from access point of view it is better park the car on the same level as the living areas. Which means it is better to have access from the top of the section. However from a cost perspective it would be cheaper to build the garage under the house, but you will have to walk upstairs every time you want to enter the house. You have to decide what is most important to you. Especially older people would prefer level access.(from the top) A split level home can be a great option as only half a flight of stairs will need to be taken.
As discussed at (1), building on a sloped site is more expensive compared to building on the flat. When you build a house on poles, you usually also have to build the outdoor areas on poles and the this will require balustrading, resulting in extra costs. If you prefer to retain the site and have a bottom story under the house, you will have to pay for retaining walls and engineering. These are usually expensive. Steep sections are usually cheaper to purchase and the steepness can be used as a bargaining tool. Building on a slope also take longer compared to flat land as the site access is more difficult. If the site is not too steep you may want to consider building a split level home. This can be cheaper as earthworks will be less compared to 2 full stories.
Geotechnical issues are the site conditions relating to site stability. Land in New Zealand can be unstable due to the nature of the land and the exposure to earthquakes.
Stormwater runoff can also be a big issue on slopes. Running water can undercut foundations or cause slips. So called "No building zones" can apply as they are unable to provide a safe foundation. These issues need to be looked at by a professional.
Every building site is subject to council planning rules. A restricting rule can be the “Daylight angle” rule. This rule can restrict you from building over a certain height without approval from the neighbours. This rule can force you to excavate the house into the ground. Other rules may apply like front yard. This rule restrict you how close you can build the garage to the front yard boundary. Private covenants can also be in place restricting your options. Easements may be present on site so its pays to check on the title before buying the section.
Car parking may have to be provided for a new house by council law. Providing these carparks can be expensive because you may have to build car decks or retain the land to provide flat car parking. This will add extra costs to your house. Usually you have to provide at least 2 car parking spots.
Not every site is so clear defined and there could be other issues not listed here.So it would be wise to involve an expert early on in the (pre)design process as mistakes can be very costly later. Feel free to contact me if you have any questions or comments. I am happy with any feedback or looking forward to helping you to find the right hillside section.