If I were to scratch build a helicopter, as the manufacturer, would I need a pilot's license to fly it?
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:

Mosquito Helicopter

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.