Op your mum is absolutely right about getting started now. You have plenty time to get architect recommendations and then visit their past projects/take up references etc. - a proper beauty parade!
You can then take time getting the plans right before rushing to make an application. You have to learn how to.read plans to understand what you are getting. Don't rush because it is much cheaper and easier to make changes on paper. Leave plenty time for different options to sink in and imagine how you will live in the new space.
Once you have really thought it all through then fix your plans and move to the planning application. Later the architect will add more detail and again take time to.absorb all of this because changes later will cost more time and money. Dont go out to tender unt you are sure you are happy... Then go for it!!!!
Most people dont understand what they are getting then fiddle with the plans on site....... Grim. Starting now and taking your time will make the process much smoother.
I had to fiddle with the plans on site - what we ALL assumed was a solid wall was rubble when the skin came off so we could not rest the steels on it BUT we adapted and the place has more character for it.
Google sketchup is great AND use the ikea online planners to play with all the spaces it will let you ...