The message
from Chris Nellist
contains these words:

before we bought our current house (I did the checks and
conveyancing), we learnt that the previous owners had failed to obtain a
completion certificate for the conversion of a garage to residential use. I
contacted the council and they said it was too long ago for them to have
any objections now. It was less than 10 years, can't recall exactly how
long off-hand. In severe cases you can ask for a letter of comfort.

If you ever want to sell that property, the buyer (or his mortgagor)
is likely to want a letter of comfort; which should properly have been
applied for by your vendor, because they should have paid the costs.
LOC's are not reserved for severe cases and I've found no problem
getting it on the two occasions we requested one.

Janet.