Lou,
Find out if the mod you want, or a mod, has been accomplished/approved before on an EASA aircraft in an EU state after and before 28/09/03. If a mod was approved in a Member State prior to that date it will probably be "grandfathered". Have a look at the EASA web site, example below:
"EASA.A.S.01625 n/a Beech A36 Replacement of existing KI-
525A HSI system with new
KI-825 EHSI system
20/02/2006"
Follow:
http://www.easa.europa.eu/ws_prod/c/...Cs/STClist.pdf
From the STC website:
Supplemental type certificates - Lists
The following lists compile EASA Supplemental Type Certificates issued since 03/06/2004 by EASA.
'Grandfathered' STCs for which the agency assumed responsibility on 29/09/2003 and issued prior to that date are not included at this time.
Please note that the list is not yet comprehensive and will be frequently updated.