Joe wasn't saying that all minors require a design authority.
Anyone can apply to EASA for a minor mod, just as in the UK, they used to apply to the CAA regional office. Now, if EASA agree it's a minor, they normally task it back to the NAA (UK CAA etc) for technical assessment. There is more paperwork involved than under the old CAA minor route, and the cost is a bit more - the current fee is EU250 for a minor on a light aircraft under 2000kg.
For more information on classification, you can look in the AMC and GM to Part 21 at
Agency Measures | Acceptable Means of Compliance and Guidance Material
There are some tables in AMC21A-91 which guide you through the decision making process.
If you need a Major mod approved, you can only do this through a Part 21J DOA (Ok - EASA have Alternate Procedures, which are used alot in France as there are very few DOA's, but EASA don't like to use them and will positively attempt to dissuade applicants).
For the priviledge of holding a DOA, you have yearly audits - ours are done directly from Cologne, and we pay surveillance fees of EU2800 per year. (Our speciality is avionics). For minor changes, we are left alone to classify, write and approve our own mods. For majors, EASA may keep the task in-house or delegate it back to an NAA (and this may not necessarily be your own NAA!) Unfortunately EASA fees for Majors are quite a bit higher than we used to pay the CAA - EU1000 for a simple STC such as clearing the AFM supplement for EHS Mode S on a single turboprop such as a PA46.
The system is certainly more onerous than the old CAA minor and major route, but if you stick to the rules, it does seem to work OK.
Regards