There are many aspects of building design that need to be done properly. Put simply, it is worth spending the time and money on getting a good design on paper, rather then having to deal with the expensive consequences of fixing it when building, or even having to live with a poorly designed building.
Here is a checklist of the most important design aspects for a successful home:
1. Site plan - sun angles, winter, summer; prevailing winds (eg sea breezes), hot northerly, or cool southerlies (this is for southern Australia); size of site, other site constraints eg building setbacks
2. the number of rooms, and their intended uses
3. style of home - exterior look, internal flow for example open plan, construction method (eg single or double storey), cladding of home - brick veneer, weatherboard, modern designer boards etc.
4. 'features' of the home - 'designer' kitchen, multiple garages, granny flats, work/study spaces etc.
5. Budget. I've put this last, but it really dictates all the above, and in fact it is one of the areas that demands a lot of time, effort and research, and perhaps is the one that many people fail to get right.
As you can see there are many aspects of good building design to get right. A good design contributes to the 'livability' of the home, a poor design can mean a cold, dark home in winter, or a home with poor airflow in summer. Good designs minimise noise, allow light and warmth into the home.
Many many homes are poorly sited, and have the orientation wrong. It usually comes from buying a project home from a plan, and putting it on a site that it doesn't suit. I don't wish to be too critical here, just merely making an observation. Sometimes people are overly concerned with the street view. This usually means having living spaces and living room windows facing the street. But what happens if the house front is facing south? Well you don't get any light or warmth into the living space in winter. A better design has the living spaces on the northern side of the home, and carefully designed street view that incorporates the sleeping and other non living areas into it. Perhaps with landscaping if privacy is an issue.
Another issue I have seen is the garage placement. Why would you place a garage in the sunniest corner of the house? It doesn't make sense! Good design eliminates these problems.
So spend your time and your money wisely. Engage an architect or a skilled building designer who is familiar with your area - especially if it is your first build or renovation. The pay-off will be a much better designed home that will suit your needs for now, and perhaps well into the future.