Firstly, there must be sufficient foreign quota in the building. The foreign quota is defined under the Condominium Act that states up to 49 percent of a condominium area can be foreign-owned.
This figure applies only to the combined saleable unit area and therefore does not include the communal space such as a lobby, swimming pool or any other facilities. The remaining 51 percent of the same area, therefore, must be Thai owned. If a foreigner is buying a resale condominium unit from another foreign, then this will keep the foreign quota the same.
However, should a foreigner be buying from a local then a check if there is sufficient foreign quota to complete the transaction is required at the building’s juristic office who keep tabs on this figure? For off-plan purchases, the developer will keep a record of the foreign quota.
The second requirement relates to the funds used to purchase the unit. Any foreigner investing in a property in Thailand must bring their funds into the country as foreign currency which is then exchanged into Thai baht in Thailand to be transferred to the developer or to the existing owner.
Source : https://www.ddproperty.com/en/property-guides/requirements-for-foreigner-property-ownership-16921