Korth Edicts

Roughly 1000 years ago, Khorvaire was united after the formation of the Kingdom of Galifar. The dragonmarked houses and The Twelve couldn't challenge the power of this united kingdom, but Galifar recognised the threat they could pose, and that it would be better to have them working together than against each other. The Korth Edicts were the solution to this, a set of laws that gave the dragonmarked houses certain rights, but also imposed restrictions on their political power.
The Korth Edicts forbid any member of a dragonmarked house from owning land, and places limits on how large house enclaves can be. It also forbids dragonmarked houses from assembling military forces, with a special provision for House Deneith, which is able to assemble military forces for the explicit purpose of mercenary contracting. Furthermore, the Korth Edicts forbid any member of a dragonmarked house, or the nobility of Galifar, from being bound to each other. No member of a dragonmarked house, or the aristocracy, can marry into the other without one of them forfeighting all of their heritage and rights, becoming a member of their new family exclusively.
Unable to own land, the edicts dictated that the Houses must rent the land their enclaves are built on from the crown. Prior to the Last War, this rent would have been paid to the treasury of Galifar, in addition to taxes, but following the dissolution of the kingdom, the ruler of the nation the enclave resides in is treated as "the crown" for purposes of the Korth Edicts.
With the Last War dividing the continent, and twelve sovereign nations being recognised by the Treaty of Thronehold, it is questionable whether any of the nations has the power to actually enforce the Korth Edicts. At the moment, The Twelve are largely testing the limits of the Korth Edicts, they aren't being violated at scale, but they are definitely not being upheld. There are blatant violations, such as Aundair granting the island of Stormhome to House Lyrandar. There are also unusual cases, such as House Vadalis and the Eldeen Reaches. Following the secession of the Reaches and their lack of a central government, Vadalis negotiated with the Wardens of the Wood, granting them the land their holdings were built upon in exchange for supporting and maintaining the local infrastructure. This is a violation of the Korth Edicts, but the Eldeen Reaches didn't exist when the Korth Edicts were written, and the house provides a service that the Wardens of the Wood have no interest in providing.
House Tharashk also owns land in the Shadow Marches, in particular within Zarash'ak. The Shadow Marches were never a part of the Kingdom of Galifar, and is not considered a sovereign nation by the Treaty of Thronehold, so Thrashk would tell you that it's holdings there are not in violation of the Korth Edicts.