In Hong Kong, is it not necessary to rent out a flat by go throught agent, you can prepare tenancy agreement yourself and sign with tenant, just to ensure your tenant will pay rent to you timely and keep your flat in good condition. Usually, tenant will not ask you to provide letter from bank regarding premission of leasing property. No one will care your premission, but if your bank find out you lease out the property, they may revise the mortage loan term and charge you higher interest rate. Once thing, in case you have rental income, you must informed the tax department by filing property tax return, and individual tax return, and declare you have rental income. Tax department will not ask you to provide bank letter regarding premission of leasing flat. If you do not report rental income to tax department, that means you are not compile to tax ordinance, you may have problems in future.
For me I think it is very simple, just to purchase a property, and inform bank it is used for leasing purpose, ask them to offer you mortgage loan interest rate = HIBOR + 05 or 4.85% whichever is lower will be used. Also, when you receive rental income, report to tax department. This is what I am doing the similar thing.
The different is, in order to save tax, I use a limited company to hold all properties.
If you have any queries, love to share experience with you.