Are you planning to let your London property any time soon? Then, the first job you need to undertake is a comparative study of rents in your neighbourhood. It will give you a fair idea about the average rent yield in the locality on the basis of which you can consult an estate agent to take your plan forward.
In some cases, your estate or lettings agent should advise you on the rent you should charge based on comparable properties in the area, and the yield you can expect to achieve once all costs and fees have been taken into consideration. If you are convinced with the rent yield, only then you should let your property.
If you are renting your property privately you will need to research the rent being asked on properties of a similar specification and location – online property portals are the easiest way to achieve this although be aware that rentals being asked by an agency are likely to be higher than those from private landlords so be careful that you do not over-value your rent.