You can only offset the interest on a mortgage against rental income for income tax, not repayments, so that is worth considering when remortgaging. Also you'd need to keep track of any maintenance expenses, insurance costs and fees to offset.
Insurance need ot be informed and if you need some cover for whatever contents remain (including carpets, white goods etc) and buildings. There are fire regulations for soft furnishings and upholstery , plus an annual gas safety inspection and one off electrical inspection and now an energy rating to organise.
Letting agencies charge between 5%pcm (for finding a tenant and collecting rent/deposit) up to 15% for full management (organising small repairs, inspections etc). You may also be charged separately for per contract and inventory.
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