As others have said its entirely down to EASA. The process is:
Form 32 goes to EASA detailing the mod.
Easa decide MINOR or MAJOR (note any mandatory Flight Manual Supplements will almost certainly be MAJOR - apart from the FMS for IFR approval of the GNS series of Garmins GPS's for which there is an exemption)
EASA provide a project number to CAA / NAA.
If MAJOR - Part 21 Design Org
If MINOR, datapack produced locally and sent to CAA / NAA for approval.
Once approved, an exemption can be requested from CAA/NAA to allow realease back to service pending EASA technical visa.
CAA/NAA send approval details to EASA who issue the Technical Visa.
From experience on avionics I can say that you can push the boundaries quite far in certain areas (we have mods on file for 1 x audio panel, 2 x NAV/COM/GPS, 1 x ADF, 1x DME, 1X XPDR, TCAS and stormscope all under a MINOR), but on the other hand try and get a GTX330 Mode S transponder through with a FMS telling the pilot not all DAPS are available on the aircraft and it will be MAJOR
It also comes down to how you word the title and description of the mod. Failing to put a simple statement in when applying may easily turn an simple MINOR into a MAJOR. You get to know the wording that keeps em happy once you have had enough kicked back at ya!!!