As a general rule there’s no use buying a plot of land without planning permission, but there are always exceptions...
Planning permission refused
Speak to the planning department and find out why it was refused and establish whether an alternative application may be successful.
No planning permission
Consider an option agreement with the owner that obliges them to sell you the land if you manage to get planning approval. You don’t have to own the land in order to apply for planning permission.
Planning permission previously granted
If the time has elapsed, check with the planning department to see if there is any reason why it can’t be renewed.
Plot already has outline approval
The principle of building on the land has been established, but there is no guarantee that you will be able to build what you want. You will need to apply for detailed or full planning permission before being able to build.