Thanks for your question - I am Sam and I am one of the UK tax experts.
If the rents will be in excess of £2500 a year - then you will need to register for self assessment and I have provided the link here for you - this then advises HMRC that you have the rental income and ensures a tax return is issued to you (and the first one will be after 05/04/2016 - for the tax year 2015/2016 so you will not have to complete one for this tax year - which will be issued after 05/04/2015)
If the rents are less than £2500 a year - then just telephone HMRC as soon as the rental income begins and they can arrange to either adjust your tax code to take this into account (if you have a PAYE source of income such as employment) or accept the figure annually to assess tax due under PAYE. Or you can write in after each 5th April has cased enclosing your P60 ( if in employment) and details of the rental income.
Their contact details are
Telephone: 0300(###) ###-####/span>
Find out about call charges
8am to 8pm, Monday to Friday 8am to 4pm Saturday
Closed Sundays and bank holidays
I would also advise ringing as soon after 8am or as close to 8pom, which ensures a chance of getting through to an officer quickly, and you can always ask them to confirm what you have discussed in writing if you would like to.
Let me know if you have any follow up questions