My wife and I (both Brits) have just had approved our fifth annual extension (at Chiang Mai) but we were given a  polite warning that the paperwork backup would change for next year.  Specifically, part of our bundle was a copy of a Marriage Affirmation Cerificate issued by the-then British Consul in Chiang Mai.  That did the job in 2014 and we were told then by Immigration that we would only ever need it done that once, with a recopy simply required each subsequent year - which is how it has worked, until now.

We are now advised that we will need a new Affirmation Certificate, bearing an updated Consul / Embassy stamp, for next year.  The cost in 2014 was 3,000+ baht and I don't know what the fee might be now.  I also don't know if it would require a personal visitation to Bangkok.

Is this a new requirement?  Has it always been there but not until now been enforced? 

The logic escapes me save for the possibility of a divorce in the intervening period but our annual applications are founded on a common address (for a property for which we have a 30 year lease) - for which an Immigration officer might visit to confirm anytime, a working bank account in joint names - in addition to the dedicated 800k requirement bank account, as well as common exit and entry stamps in our passports in and out of the country.

Thanks in anticipation.  

The embassy stopped doing the affirmation 3 years ago, Your marriage certificate has to be legalized in the UK. The UK embassy is not not involved in getting it done. Info is can be found in the following.

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/714089/Legalisation_info_June_2018.pdf