Aviat husky pilot reported an near midair collision with a light twin on approach.

Attributes
ACN	1109782
Time
Date	201308
Local Time Of Day	1201-1800
Aircraft 1
Make Model Name	A-1 Husky
Operating Under FAR Part	Part 91
Flight Phase	Landing
Route In Use	Visual Approach
Flight Plan	VFR
Aircraft 2
Make Model Name	Small Aircraft
Operating Under FAR Part	Part 91
Flight Phase	Landing
Person 1
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Private
Experience	Flight Crew Last 90 Days 27 Flight Crew Total 298 Flight Crew Type 211
Events
Anomaly	ATC Issue All Types Conflict NMAC
Miss Distance	Horizontal 300 Vertical 125

Narrative:

I was asked to extend my downwind leg and the controller would call my base. I [was] informed to start my base and controller apologized (he extended my downwind too far.) I was cleared for 28R. I was on short final 200 ft from the threshold when a twin engine aircraft came from my 4 o'clock position almost 150 ft below me. It appeared he was cleared to land on 28L and switched to 28R. The plane I was flying was doing 65 mph indicated. I contacted tower [and] explained I had traffic 150 ft in front of me landing and I was going around. Controller advised me to do a right 360 and I was number 1 28R. Upon starting my right 360 I had to apply power and climb due to buildings. My climbing 360 turn brought me within 1;000 ft of a champ that was on his downwind leg. I was not advised of traffic. I continued my 360 and made a touch and go. Upon landing no contact was made by tower to me other than the standard taxi instructions. I am filing this report by advice of a senior flight instructor at field.

Original NASA ASRS Text

Title: Aviat Husky pilot reported an NMAC with a light twin on approach.

Narrative: I was asked to extend my downwind leg and the Controller would call my base. I [was] informed to start my base and Controller apologized (he extended my downwind too far.) I was cleared for 28R. I was on short final 200 FT from the threshold when a twin engine aircraft came from my 4 o'clock position almost 150 FT below me. It appeared he was cleared to land on 28L and switched to 28R. The plane I was flying was doing 65 mph indicated. I contacted Tower [and] explained I had traffic 150 FT in front of me landing and I was going around. Controller advised me to do a right 360 and I was number 1 28R. Upon starting my right 360 I had to apply power and climb due to buildings. My climbing 360 turn brought me within 1;000 FT of a Champ that was on his downwind leg. I was not advised of traffic. I continued my 360 and made a touch and go. Upon landing no contact was made by Tower to me other than the standard taxi instructions. I am filing this report by advice of a senior flight instructor at field.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.