A C172 PLT IS GIVEN AN APPROVAL BY TWR TO FLY AN INST DEP FROM STS; BUT AFTER CONTACTING CTR; THE PLT WAS TOLD HE WAS NOT APPROVED.

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WHILE IN VFR CONDITIONS; I HAD RECEIVED A POP-UP IFR CLRNC FROM ZOA FOR THE ILS RWY 32 APCH INTO STS. VFR CONDITIONS PREVAILED IN THE AREA; WITH STS COVERED WITH COASTAL STRATUS. STS ATIS RPTED VISIBILITY 1 MI IN MIST AND 100 FT OVCST. I WAS VECTORED TO THE FINAL APCH COURSE AND PROCEEDED INBOUND ON THE APCH. I TOLD ZOA I WISHED TO FLY THE SANTA ROSA 5 DEP UPON MY MISSED APCH. ZOA REPLIED WITH 'ROGER.' I WAS SOON TOLD TO CONTACT STS TWR; WHICH I DID. I THEN TOLD STS TWR THAT I WISHED TO FLY THE SANTA ROSA 5 DEP IN LIEU OF THE PUBLISHED MISSED APCH PROC. TWR REPLIED WITH 'APPROVED.' I COMPLETED THE ILS RWY 32 APCH; WITH THE ARPT IN SIGHT AT ABOUT 700 FT MSL. I CONTINUED TO THE PUBLISHED DECISION HT AND BEGAN THE SANTA ROSA 5 DEP. TWR HANDED ME OFF TO ZOA; AND I RPTED I WAS ON THE SANTA ROSA 5 DEP. CTR TOLD ME I WAS REQUIRED TO GET APPROVAL FROM CTR PRIOR TO EXECUTING THE SANTA ROSA 5 DEP IN LIEU OF THE PUBLISHED MISSED APCH. AT THE TIME; I ASSUMED CTR HAD APPROVED MY REQUEST; WHEN THEY REPLIED WITH 'ROGER.' MY MIND SET WAS REINFORCED WHEN TWR TOLD ME MY REQUEST WAS 'APPROVED.' CTR ALSO TOLD ME THAT IT WASN'T A PROB THIS TIME; BUT IN THE FUTURE; FIRST OBTAIN AN APPROVAL FOR SUCH REQUESTS. I NOW REALIZE THAT 'ROGER' ONLY MEANS THE CTLR RECEIVED MY XMISSION AND IT IS NOT A YES OR NO ANSWER OR APPROVAL TO PROCEED AS REQUESTED. MY ERROR WAS UNINTENTIONAL AND IN THE FUTURE; I WILL CLARIFY WITH THE CTLR WHEN I HAVE A SIMILAR REQUEST OR SIT OR QUESTION ABOUT A PROC OR CLRNC.