My garden is approx 3 years old, I've planted up 2 main beds over the 3 years but want to eat into the lawn and create 2 new planted areas the first with a lavender hedge either side of a path 2) is a triangular shaped bed to cut the corner of a square side to the garden. Am I ok to plant (shrubs/ perennials) now?
Only if you want to be a slave to the hose/watering can. I would wait until autumn when the soil is still warm but moist - the plants will soon get their roots down. Having said that, if you think you can do a better job than the garden centre at keeping the plants watered (they'll still be in pots there) then go for it!