Now you can start to explore and sketch out how you can achieve your aims. Again, dream more than worry about detail, an architect could draw it up accurately and as close to your vision as possible
Think in 3 dimensions - don't end up banging your head on the ceiling or needing a rope ladder. Allow for wall thicknesses and floor depth. Relate to views, sunpaths and prevailing weather, and of course your neighbours, think about how your building will relate to the garden.
Take note of ground levels: you will need to dig out higher spots and it will cost extra to make those areas damp-free, conversely you will need to build up from low spots, with extra materials costs. You may achieve a better solution by going with the natural ground levels rather than altering them.
Keep an eye on your budget - in the Southeast (not London) reckon on approximately £1,600 per square metre of new floor area all in, excluding land. A small house will have about 90m2 floorspace.
Draw to scale, 1:100 will do, use graph paper if you need to, but don't be mesmerised by the grid, walls can angle and curve. You will begin to imagine what your building will look like - especially in section and elevation - and you will need to think about proportions and wall finishes to make it look good.
Take time over this, once into the next stage it is harder to go back on the basics.
Your choice of materials and finishes will be a major influence on your building. How those materials 'work' to filter the environment , be jointed to other materials, hold up themselves and other elements, will determine the look of the result.
At this stage you do not need to have detailed knowledge of how your project will be made, the priority is to make the spaces right for how you want to live in them. Specialist manufacturers & suppliers may provide information for integrating later. You do need a general idea of what is feasible - it helps if you have watched things be built before.