The quick answer to that question is...NOW!
Seriously though, a lot of people contact me over the Easter break, when the weather starts to improve, hoping to have the garden designed and built ready for the summer...this time frame is just too short unfortunately.
To enable the client and designer to work through a thorough brief and maximise the use of the space the process can easily take 3 months. This may sound like a long time but I have listed all the phases in the process as follows;
Concept design (presented up to 5weeks after survey)
Revised concept design
Drawings issued to landscapers for pricing
Plant mood board
Plants sourcing/planting design issued to landscapers
Most good landscapers will be booked up for at least 5 months, especially during the summer months.
Ideally it's best to get in touch with me during, or towards the end of, the summer with the intention of having the garden designed and built ready for the following summer.
As with everything, there are always ways round things and the process can be condensed but 3 months is the ideal scenario.