To furnish or not to furnish – it's a question many landlords ask. Surprisingly, the law has no particular opinion: since 1988 there has been no difference between furnished and unfurnished property in security of tenure for tenants, and in fact, there's no legal definition of "furnished" or "unfurnished".
There are financial implications though: Council Tax isn't payable on unfurnished property for the first six months of a void period. Landlords of furnished rentals can claim 10% depreciation allowance annually. You won't be able to claim both on the same property.
What's actually included in furnished accommodation can vary wildly, both in terms of tenants' expectation and landlords' provision. Even unfurnished property will generally have carpets and some white goods (fridge, cooker, perhaps a washing machine). Prospective tenants of furnished property will probably expect to see these, plus sofas and beds as a minimum. Some furnished lets include everything up to crockery and bedding.
If you do supply furniture, it will need to meet fire safety standards. Electrical and gas appliances will have to be checked over at the beginning of a tenancy and appropriately serviced. Obviously this can add to costs, and to time spent on administration at the beginning of a new tenancy.
What you choose to do will depend on the kind of property you have and the kind of tenant you're hoping to attract. Many tenants prefer to bring their own furniture, and (I write this as some one who once lived with four sofas in my living room, as my landlord refused to remove his and I liked mine better) too much furniture can be as bad as too little. Tenants of unfurnished properties may be more stable too: it's hard to do a midnight flit if you have to take a double bed and two sofas with you.
If you're renting to families or young professional couples, your tenants will probably be bringing at least some of their own furniture, and may well prefer an unfurnished property. If you're renting to those just leaving home or setting up on their own, furnished might be more useful.
If you have a few properties, be prepared to move furniture about, and perhaps store it when not needed. And if you're just starting out with a new property, you can always advertise as "furniture supplied as required" – and then purchase as and when (and if) needed.
Finally, whatever you decide to do, you and your new tenants should run through the inventory of the property before they move in, confirming any existing damage in writing, and photographing it if possible.