My FBO routinely refuels aircraft with 100LL while they're parked inside hangars. I have complained about this being a dangerous practice. My previous FBO required aircraft to be at least 10ft outside the hangar for refueling. Are there any regulations or advice on this?
Your previous FBO is correct. According to FAA Advisory Circular 150/5230- 4B Aircraft Fuel Storage, Handling, Training and Dispensing on Airports:
Standards The FAA uses the standards contained in the most recent edition of National Fire Prevention Association (NFPA) 407, Standard for aircraft Fuel Servicing. NFPA 407 provides a standard for the storage and delivery of aviation fuel in an airport environment
NFPA 407, on the other hand requires outdoor fueling.
5.10 Aircraft Fuel Servicing Locations.
5.10.1 Aircraft fuel servicing hall be performed outdoors...
5.10.2* Aircraft being fueled shall be positioned so that aircraft fuel system vents or fuel tank openings are not closer than 7.6 m (25 ft) to any terminal building, hangar, service building, or enclosed passenger concourse other than a loading walkway.
A revision of the same standard also looks the same, with some differences in wording. So, the aircraft fueling should be done outdoors. Its doubly important if the hangar/airport where your aircraft is located also follows NFPA 407.
(b) Each certificate holder must establish and maintain standards authorized by the Administrator for protecting against fire and explosions in storing, dispensing, and otherwise handling fuel (other than articles and materials that are, or are intended to be, aircraft cargo) on the airport. These standards must cover facilities, procedures, and personnel training and must address at least the following:
(2) Public protection.
(3) Control of access to storage areas.
(4) Fire safety in fuel farm and storage areas.
(5) Fire safety in mobile fuelers, fueling pits, and fueling cabinets.
(6) Training of fueling personnel in fire safety in accordance with paragraph (e) of this section. Such training at Class III airports must be completed within 12 consecutive calendar months after June 9, 2004.
(7) The fire code of the public body having jurisdiction over the airport.
This AC on the topic also does not really talk about fueling indoors or otherwise lend any advice about it.
It may just be that your local airport procedures called for not fueling in hangers. There is an interesting debate on the topic here. That discussion would elude to the fact that there are problems that can arise and some cite cases of it causing an issue. But there are no hard regulations about fueling indoors.