The short answer - and we know it's probably not the one you want to hear - is yes. If you've been living in a property on which you had a residential mortgage, but are intending to move out and let the property instead, you will need to inform your mortgage lender. If you're still living in the property and have simply taken in a lodger, you won't normally need to inform your mortgage lender of that.
Most residential mortgage terms require your lender's permission to rent out the property instead of living in it yourself. If you don't consult your lender, you may be in breach of your mortgage's terms, so do read the small print before you decide what to do. As most lenders will charge you a fee to even answer the question, it's tempting to keep quiet and hope they don't notice. But there are many ways in which lenders can discover you're no longer resident in the property (mail returned by tenants is the most likely scenario), so honesty has to be the best policy. It's not worth the risk of losing your property
Once you've paid the fee to have your case considered, most lenders are sympathetic to letting, but be prepared to argue your case: go armed with figures of what your rental income is likely to be, and proof that this would more than cover your mortgage plus maintenance costs and any void (empty) periods. Some lenders will simply increase your interest rate by 0.5%-1% and give you formal permission to rent out the property. Others will insist you switch to a buy to let mortgage; this will again probably be more expensive than a residential mortgage, and you may also incur early repayment fees on your original mortgage.