Read about it here. It means you might be eligible for a tax credit, but it doesn’t restrict what you can do to your home.
Can I modify, remodel, or renovate, my historic house?
From the federal perspective (the National Register of Historic Places is part of the National Park Service), a property owner can do whatever they want with their property as long as there are no federal monies attached to the property. You can find this on our website at:
http://www.nps.gov/history/nr/national_register_fundamentals.htm
More FAQs here.