If I were to scratch build a helicopter, as the manufacturer, would I need a pilot's license to fly it while its in the experimental stage? This is in the USA, but I'd be interested in how it plays out in other countries which are different
Depends on what you build. It is possible to build a helicopter that qualifies as an ultralight, and thus no license needed.
If it exceeds the weight and power restrictions for an ultralight, then it is an experimental aircraft, that requires at least a solo ticket.
I would strongly suggest that rather than scratch build your own, an experience you might not survive, you take a look at what I'm currently lusting after:
In kit form (which technically makes you the builder), it can be had in ultralight form, with limited range.
Or in experimental form with a more powerful 2 stroke engine and larger fuel capacity.
And finally, the (sigh) gas turbine powered version.
One advantage of building the experimental kit is: as the builder, you can perform all maintenance and the annual inspection yourself.
Another advantage of going the more capable experimental route is: you can log the hours.
In Germany you cannot just start to build man-carrying aircraft. First you need to certify your development office with the LBA, the local version of the FAA. They will check if you have sufficiently trained experts on hand and that your processes are appropriate. For homebuilts, this responsibility is devolved to an association, the Oskar Ursinus Vereinigung, which will send experienced people over to check every step of your work. Also, the first flight has to be performed by a trained test pilot. They will put many roadblocks in your way to become an aviation martyr.