increasing gross weight for E-AB

The short answer is, YES.

As you seem to realize, there is no such thing as an STC for an experimental airplane, because the FAA does not issue a type certificate to begin with. Thus a supplemental type certificate would be meaningless, since there is nothing to supplement.

With an E-AB airplane a gross weight increase comes under the heading of a major modification.  In all cases you should first look to your Operating Limitations for guidance about how to deal with a major modification. In all likelihood your OLs will require you to contact the local FSDO and ask for a flight test area and flight test period (number of hours) in which to test your major mod. You would then prepare new weight and balance calculations, re-enter the Phase I Flight Test period (as per the FSDO), fly off the hours, and make the appropriate logbook entries.

Of course, we hope that there is some reasonable justification for increasing your gross weight, but that is up to you to determine.

Thank you for the explanation Dave

I agree with Dave's comments and would like to add an engineer's perspective. You haven't mentioned what airplane you are considering. Some designs could accomodate it and some couldn't. That's a 7.3% increase in GW. Is it structural? Is it payload? What will it do to the G limits? What is the change in wing loading/ How much faster will it stall at? Is the landing gear rugged enough? Are the fittings and attachments safe at the new GW. Perhaps like some TC'd aircraft you can have a higher GW but with reduced maneuvering limitations and no structural changes at all.

All of these questions need the advice of a designated engineering advisor for amature built aircraft which we now have in the EAA.

Dave and Ed,

I agree this should be based on engineering and not wishful thinking.

The manufacturer of the aircraft set the gross weight at 1230lbs. prior to the new LSA weights were known.  Several E-AB builders have chosen to set their own gross weight to 1320lbs. as a result of LSA.

I understood the process for standard category aircraft (STC for gross weight increase - Piper Super Cub comes to mind), but did not know how it would be done, or if it could be done, were the appropriate egineering review were completed for an experimental aircraft.

BEWARE: It's definitely net just a case of changing the number.  An acft is designed based upon the stress factors it can tolerate without breaking up in flight..  The wings can accomodate only so much load (weight times "G:" force).

It takes a really good pilot  (like God) to control an airplane when a wing breaks off in flight.  (Of course, it gives you a good opportunity to test the BRS chute)