Hello Mearie,
Greenhouse plastic is definitely heat sensitive, so I would allow a little extra room compared to a normal
wood or drywall combustible wall. Many folks build a
raised bed in the middle and run the rocket heat though this, so it heats the greenhouse evenly.
If you can tolerate reading some draft building code, take a look at this page for clearances that are recommended in most situations:
http://www.ErnieAndErica.info/rocketmassheaterpermitting
The barrel will be hottest, and there
should be good clearance around this and the fuel
feed, for both plastic and plants.
Yours,
Erica W