Legally, any building is considered as being a part of land over which such building is constructed. However, a building may be considered as a separated part when it is the tenant of land who constructs the building under a leasing agreement. Therefore, a foreigner may own any building on his rented land.

If your spouse is Thai, and you are planning to build a house on his/her land, you are recommended to sign a lease agreement with your spouse indicating that you are a tenant. This way, you shall have a joint ownership over such construction, not a land, together with your spouse.