The first action one must always take when wanting to let a mortgaged property (in the UK, and possibly elsewhere) is to contact your mortgage lender to let them know what you would like to do (ie short-term holiday lettings or a long-term Assured Shorthold Tenancy (AST)) and asking for their advice as to whether they will give you their (written) permission to do such.
Should your Mortgage provider give you their permission, you may well find that there will be caveats to same, one of which might be that you have to change to a different mortgage product, eg one more suitable for "letting" your property. You should be aware however that any change in mortgage product may incur charges for the change, and furthermore, that the new mortgage product night be at a different interest rate.
Should you be given written permission to let your mortgaged property, you then would have to take out a different Property Insurance product, ie for a Holiday Let [or for an Assured Shorthold Tenancy, a Tenant Let] Insurance Policy - and again, this too will, most likely, be at a different rate to your current property insurance policy.
Of course, something else you need to comply with, is completing an annual Tax Return, providing details of your income from lettings, as well as your outgoings re same +the bonus being that for holiday lettings one is able to claim for some expenditures that AST Landlords aren't allowed, but as holiday lettings have different formulae to that of AST lettings, I would suggest that you contact an Accountant with holiday lettings experience either for advice, or for them to attend to your Annual Tax Return on your behalf. OK, so that will cost you money, but their charges are Tax deductable, so if you're able to complete your own Tax Return, then you won't have to pay for an Accountant, but if you can't complete a Tax Return on your own, or without help from someone who knows exactly what they're doing, then an Accountant will be the best route for you to take.
When I first started lettings, it was as a Landlord, and at the time, my Accountant (for I used one straight away) explained to me exactly what he had done when completing my Tax Return, so much so, that when - a few years later - he died, I was able to complete my own annual returns, successfully.
In 2012 one of my properties changed from a Shorthold Tenancy to Holiday Lettings, but rather than continue doing my own Tax Returns, I turned to an Accountant who knew all about holiday lettings, and now, some 8 years later, I'm still with the same accountants, and hope to be until I'm no longer fit enough to do everything in still doing at the age of 73!
Whatever you decide to do, please remember that for those that follow it, the Law is a very good friend, but abuse it in any way, and it will be your worst enemy.
Over the years I have seen and heard about those who try to circumvent or flaunt what the Law of the Land, the Laws regarding driving, and the Laws relating to Mortgages, Lettings (all types), Tax Returns, Insurances, and other aspects of property/properties have to say, and often do, when Mortgagees fail to comply with their lenders' Rules and Regulations.
The main thing to remember, is that the property for which you have taken out a Mortgage, will never actually be yours until you have paid the very last 1p you owe your Mortgage provider, which conversely means that the latter owns every part of the property, so therefore is able to dictate to you, the borrower, the terms under which you are allowed to live in and on the property
Any changes to the said Terms - to which you signed up to with the Mortgage provider, can ONLY be changed by them, and them alone, but by the same token, should you ever decide to "do" holiday lettings without asking both your Mortgage provider AND your property insurer, you will in effect have broken the Terms of your Contract with both, and as such it would be highly likely that they might repossess your property (unless they and you are able to come to an arrangement, which may include changing the products, as mentioned previously)!
Personally, I would always be open with those to whom I owe money, but then, that's me
