Courtyard garden and determined non tray user

[NickettyNacketty]

we are moving to a new house soon which has a completely paved back garden. Next door has a small border, a tip of a courtyard and judging by all the enormous poos out there, a huge dog. Last house in the row again has a paved yard. We back onto a school playground, all tarmac.
Two of My cats are used to popping out into the bushes to do their business and one only uses the tray. What will they do now? Should I make a cat toilet/ sand tray in a corner behind a large pot plant?
The front garden is tiny and is on a busy road so I wouldn't want them out there really.