PLT OF AN ATX PIPER PA31; DRIFTED OVER THE CTRLINE OF THE RWY WHILE FLYING THE DOWNWIND LEG IN THE TFC PATTERN AT NIGHT IN VFR CONDITIONS. THE TWR ADVISED RPTR OF HIS POS AND SUBSEQUENTLY ISSUED A VECTOR TO FINAL.

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WHILE APCHING SEA/BOEING FIELD I WAS HANDED OFF FROM SEA APCH TO BFI TWR AT 1 MI N OF ARPT; I WAS INSTRUCTED BY BFI TWR TO CONTINUE ON THE DOWNWIND LEG FOR RWY 31R; AND THAT TWR WOULD CALL THE BASE LEG. WHILE I DID NOT HAVE THE ARPT IN SIGHT; I ASSUMED FROM THE TWR CTLR'S INSTRUCTIONS THAT I MUST BE ESTABLISHED ON THE DOWNWIND LEG. I MAINTAINED A HDG OF 130 DEGS AND WAITED FOR THE CTLR TO CALL MY BASE TURN. A MOMENT LATER; THE CTLR INFORMED ME THAT I HAD CROSSED OVER THE CTRLINE OFF THE RWY; AND INSTRUCTED ME TO EXECUTE A L 180 DEG TURN; FOLLOWED BY RADAR VECTORS TO FINAL. UPON LNDG I WAS INSTRUCTED TO CALL THE TWR. I BELIEVE THAT MY UNFAMILIARITY WITH THE ARPT AND THE PUGET SOUND/SEA AREA CONTRIBUTED GREATLY TO THE EVENT. I FURTHER BELIEVE THAT THE CTLR'S VERBIAGE WAS SOMEWHAT MISLEADING: LEADING ME TO BELIEVE THAT I WAS PROPERLY ESTABLISHED ON THE DOWNWIND LEG. FOR MY OWN PART; EVEN THOUGH RADIO CONGESTION WAS HVY; I SHOULD HAVE NOTIFIED TWR THAT I DID NOT HAVE THE ARPT IN SIGHT. FURTHERMORE; I BELIEVE THAT THE TWR SHOULD HAVE RECOGNIZED THAT I WAS NOT FLYING A STANDARD DOWNWIND DISTANCE FROM THE RWY LONG BEFORE THE ACFT CROSSED OVER THE RWY CTRLINE.