Watching the first episode of Dangerous Flights, a ferry pilot has aftermarket fuel tanks fitted inside the cabin of an old Merlin IIB:
What are the regulatory requirements to be able to do this? / How can you get permission to fit extra tanks inside a plane?
Is there anything that needs to be specially considered when designing the tanks and valves etc?
Is this commonplace when ferry flying long-haul?
This is covered by 14 CFR 91.203:
§91.203 Civil aircraft: Certifications required.
...
(c) No person may operate an aircraft with a fuel tank installed within the passenger compartment or a baggage compartment unless the installation was accomplished pursuant to part 43 of this chapter, and a copy of FAA Form 337 authorizing that installation is on board the aircraft.
14 CFR 43 covers MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION.
Basically, it needs to be an approved installation that is signed off and approved by the FAA using a Form 337 - MAJOR REPAIR AND ALTERATION (Airframe, Powerplant, Propeller, or Appliance). The maintenance shop will take care of that aspect for you.
It is pretty common for delivering aircraft that need to cross the ocean, and there are even companies that specialize in it. Really, your only option is to purchase an already approved kit because certification costs are so high.
NBAA actually has a nice article which goes into more details, with lots of helpful links.
