The more stringent crash worthiness requirements put in place in the 90's have made fuel systems, seats and some other structural members heavier. there may be some weight savings in avionics. But all in all that is not to far off. going for a 20,000 gross weight sounds like the target build is a S-61 or similar sized aircraft. I've never flown one but I've head 10-12k empty weight depending on variant and configuration. That's with no AC though!