Can a part 103 ultralight weigh 400kg fully loaded? I am designing an electric VTOL personal part 103 ultralight.

My question is: I have 114kg for the ultralight, 100kg for the pilot and 30kg for the baggage. About 260kg is enough to be take off safely but I want to design it up to 400kg.

If I do that, will it still be accepted as a part 103 ultralight and what is the weight limit for it?
Part 103 limitations pertain to dry weight in takeoff configuration, maximum stall speed, maximum level flight speed, and maximum fuel capacity.  They do not directly address maximum gross takeoff weight.

However, weight affects stall speed, and I'm not certain how Part 103 defines its stall speed limit -- whether they specify at some minimum operating weight, or at "design weight" or some other value.  The complete regulation is available online, however, so you can read it yourself and determine if you'll be able to design for your maximum 400 kg GTOW.