I started out with a vague requirement; I wanted an office to free up a bedroom, and I wanted a room for family meals to free up the existing living room / dining room as a large living room only.
The next thing was to consider the existing house and plot to see what was possible.
It was immediately obvious that I could take advantage of the corner position and build on the right hand side. This position was away from adjoining property, avoiding crowding and potential conflict with neighbours. It avoided cutting into the gently sloping garden at the back and positioned what would become the dining room, next to the kitchen. The ground footprint available meant my requirement for two modestly sized rooms could be accommodated by a single story.
I rejected the mild temptation to go mad and build two stories (to create perhaps a very large fifth bedroom with en-suite) on the simple basis that in order for the building cost to be reflected in the value of the property, the extended house needs to remain in proportion to the plot and surrounding properties; Don't build a five bedroom mansion filling up a modest plot on an estate of 2,3 and 4 bedroom houses!
Another thing to consider early, was what was I losing? I would lose a walled court yard and the downstairs toilet window. I would also need to relocate a gas boiler whose vent would be inside the proposed extension and it was also clear that the mains electricity cable, water main and drainage would need to be moved.