If my Thai wife (who owns our house) sub-lets a room in the house as an office to my overseas company so that the company can claim a fixed place of operation in Thailand, is there any declaration to be made to any authorities in Thailand?

Your wife can rent out the room to a juristic entity, ie a company. However, since you will be working from that room you should have a work permit allowing you to work in Thailand legally. You will need either an offshore company that is recognized by the Ministry of Commerce or a Thai company as an employer. Once you have the company (or offshore company) then that company would declare the rent as part of the company's expenses, Your wife would have to declare it as income and will have to pay a progressive tax to the Thai authorities for it.

[sunbeltlegal][/sunbeltlegal]