If you do not have a buy-to-let mortgage, then living in the property will be a condition of your residential mortgage, and thus letting it out without permission is a violation of your mortgage contract. This could lead to the mortgage lender charging you additional payments, interest rates or other fees.
You are also required to obtain consent from any organisation which has regulations that apply to the property. For example, if you hold the leasehold in a flat, you may need permission from the freeholder, whether that is a private individual, a housing association, or a local authority. Failure to adhere to the terms of your lease can result in forfeiture of the lease, which will also include legal costs.
You will also need to advise and obtain consent from your insurance provider. This is particularly important as if you need to make a claim later but are found to be in breach of your insurance policy terms, the provider is within their rights to refuse to make a payment.
Each mortgage lender has slightly different processes related to consent to let applications, so contact your mortgage lender for more information. It is not likely that a lender will refuse the application without good reason, however, they may raise the interest rate on your mortgage due to the increased risk that comes from letting your property to a third party. In some cases, your lender may ask you to apply for a buy to let mortgage, which may have different interest rates to your residential mortgage.
If you obtained your property through a scheme such as Help to Buy or shared ownership, there may be restrictions on your ability to let out the property, so check your documentation from when you purchased the property.
Ensuring that the right consent is in place protects both landlords and the tenants, and so any requirements for consent should be adhered to.