When you own the condo you are allowed to rent out the unit.
As long as it is not considered a business you are basically free to rent out your condo in Thailand. Only when it comes to the intent to do this as a business the requirements under the Foreign Business Act and Foreign Employment Act must be complied with.
Note that most condos have internal rules and regulations that only allow residential use of the units by the owners and prohibit you to rent out your condo, especially with the popularity of lodging reservation websites (like Air B & B) condominiums more and more include rental restriction in their internal rules and regulations. If this is the case you could try to change the regulations first and put this on the agenda of the general meeting of the condo. Regulations of the condo are basically made and amended by the owners of the units.
Note when the condo is offered for short term rentals less than 30 days you may be legally subject to the Hotel Act, however you may be exempt but still may have to comply with its provisions, like reporting requirements to the local immigration of guest.
Note that when you rent out your apartment you are responsible for paying land and housing tax each year, and also note that rental income is subject to personal income tax under the Thailand Revenue Code.
It is recommended to make a rental agreement (the rental agreement must have written evidence for it to be enforceable by legal action).
