To become an Assessor you have to gain a certificate(stroma I think) which usually takes 4-5 days, cost is variable but usually about £1000-£1500 depending on whom you go with.
This then allows you to issue certificates for premises, at a charge between £60-£120,
There are lots of places that offer the courses, as well as lots of places/people who will do an assessment of a premises, for both its a case of shopping around for the best deal, as they both lead to the same output. Estate Agents will put you in touch with assessors, but they would probably want you to let them sell your house sell, some will offer the assessment as part of the package deal.
Have a look at doing the NDEA course, same thing but for Non Domestic properties
I did the NDEA course some 14 years ago, 4 day residential course in Birmingham.
My thinking being that to earn reasonable money you would have to do 2 or 3 houses a day, have to deal with grumpy householders, estate agents that don't want to pay, along with having to do the documentation in your own time in the evenings, and of course time and expense of traveling to multiple locations.
Where as if you have a decent sized office block / shop for example, along with reasonably up-to-date floor plans the whole job might take 1, 2 or 3 weeks to complete and earn you a couple of grand. Minimal travel, basically hours to suit.
Win win I reckon.