Have you fully considered the local market conditions? How much are other rooms being rented for in the area? (check spareroom.co.uk,easyroommate.co.uk and rightmove.co.uk for comparables). What is the condition and décor of these comparable properties like? How will you differentiate your HMO from the competition? Are there ways you can increase your rents without a massive cost outlay? Ensure you plan in any additional features or benefits to the HMO early on so that you can budget for them too.