Rather than starting to search for pre-drawn plans or trying to figure out locations where an addition could physically fit, you really should begin by carefully evaluating your specific needs & goals.
Take note of rooms or spaces you have seen that have left a favourable impression. What is it that makes those ‘spaces’ feel good to you? Use these answers to aid in the overall layout requirements.
Seriously consider whether you really need formal Dining and/or Living rooms, most people only use them once or twice a year. Perhaps they can be used as a media room, study or IT hub and converted for formal use on occasions.
Make a ‘wish list’ of your needs, goals & desires. Differentiate between needs and wants. Prioritise each item as to its relative importance. Use these lists to evaluate various possibilities / designs.
As cost is nearly always an important factor, establish as early as you can both an ‘ideal budget’ as well as a ‘maximum budget’.
Collecting images or ‘styles’ that appeal to you are great for conveying what you like.
Carefully evaluate what is appealing; is it the entire space, the materials, just the furniture, certain colours or something else?
Think beyond immediate changes that need to be done, and consider your long-term wish list. Even if you are not wanting or unable to do everything at once, make sure you have a ‘master-plan’, so you are always working toward a common goal.