First of all, you should be aware that detailed planning permission only lasts for three years now, not five. You could renew it, and then amend with a new application. Or, you could implement your existing permission (by making a token start to the work) and then seek to amend it later. You should discuss your proposed changes with planners as soon as possible. If they’re viewed as non-contentious, get an application in quickly. In the unlikely event that it’s refused, you still have the existing permission, which you can renew. If you see that for some reason the planners are now not keen on renewing the permission, consider implementing it. You can only do this if all the conditions are agreed with the council first, and if you start the works – say by digging a foundation trench – then get the planners to confirm work has started. Effectively this keeps the permission alive indefinitely, but doesn’t deal with your proposed changes, of course. The key is to maintain the position of strength that you have right now, ie. a valid planning permission that is at least close to your ideal scheme.