Fairly straightforward, Jill..................
http://www.planningportal.gov.uk/permission/responsibilities/planningpermission/ldc
Click on the online application link.
Any PP, or in your case LDC (lawful development certificate) applies to the use of the property regardless of ownership. If it's lawful, then it is so for whoever lives there. That is, as long as there are no particular restrictions or conditions..eg Agricultural use.
It would be interesting to know how the original use was established (by the relative concerned.) Be aware though that, if it is unlawful, the current use could not be allowed.