The positioning of rooms in your layout should be such that you allow sunlight into as many spaces as possible, your views are showcased, and visual privacy is provided from neighbours or unsightly views. Never have utility-type spaces on the south, or in the way of a view.
Make sure your place doesn't kick you out in years to come because of changing needs, such as mobility issues. Ideally a single storey, or everything you need on the main floor (guest rooms above). Alternatively, spaces that later can be converted easily.
Your furnishings are critical to determine proper room sizes, as well as appropriate window locations. Draw your floor plans to scale, & always include furniture; otherwise you won't know if your floor plans work (furniture fits, while providing proper 'circulation' space).
Your addition should (1) visually blend-in, (2) integrate & flow with the original, and (3) enable the original effected rooms to be fully utilized & appealing. Your addition should be seamless & harmonious. Otherwise, the overall appeal and value will suffer.
In determining your needs, have a critical look at what you actually use. If something only gets used a few times a year, consider eliminating it. Either save your money, or build some character into your project. A dining room is an common example.
Natural light is extremely important to our sense of well-being, yet many designs fail to consider it at all - instead ending up with dark & shadowy interiors. Carefully consider the arrangement of your rooms, as well as the placement & size of your windows.
Countless 'details' will impact your convenience and your sense of satisfaction; enough counter space EACH side your stove, sink, & fridge; a place for your BBQ close to the kitchen; the right size closet at your entry; not having to close one door to open another; a convenient location for your swim towels; ...
