Can I make a planning application if I don't yet own the property?

Yes you can. You will just need to make sure that you sign certificate B on the planning application form and serve notice on the owner of the property.

Certificates can be found on the Planning Portal website. Once on the Planning Portal web site, scroll down to point 24 of that page. You will find there a list of the different type of certificates, an explanation of when they should be used and the relevant forms for each certificate that you can use.