Would you drive a BMW and take Insurance for a Fiesta? The principle is exactly the same. You cannot insurance a coach house as a detached house, or a flat, or as a maisonette. It is a Coach House, and should be specified as such in the property type section of your policy.
Failure to Insure your coach house, as a coach house, means your Insurer is not obliged to pay you out in the event of a valid claim on your policy. 3rd party Leaseholders using garages underneath your property present added risk to the insurer - so its vital that they know you are responsible as a freeholder, for insuring all of the garages, and protecting you legal liability towards the leaseholders.
These are material facts that must be disclosed when you buy insurance, and you must not assume that your insurer will know what you mean, because at Home Shield Insurance - we deal with customers everyday who thought their insurance was valid - but wasn't!
Insurance company's are not charity's, and if they can avoid paying your claim - they will do. The responsibility to ensure your Insurance is correct is yours - not theirs.