majority a tenancies require rent to be paid in advance thus the tenant(s) may be entitled to a refund of rent should they comply with the section 21 notice and give up vacant possession of the let property at a period of the tenancy which they have paid in advance for. How do I calculate a refund of rent? As required by the section 40 of the Deregulation Act 2015 an amount of repayment which a tenant is entitled to is to be calculated with the below formula. FORMULA: R multiplied by D divided by P – R is the rent paid for the final period; – D is the number of whole days of the final period for which the tenant was not in occupation of the dwelling-house; and – P is the number of whole days in that period.A known technicality which we see time and time again when checking the validity of a section 21 notice that a landlord or agent has drafted and served themselves is the incorrect expiry date (date of which the tenant(s) must vacate the property after) being inserted on the notice. Where a section 21 notice was served to end a periodic tenancy, the notice must have required possession after the last day of a period of tenancy in compliance with section 21(4)(a) of the Housing Act 1988. Section 35 of the Deregulation Act 2015 brought amendments to the Housing Act 1988 which came into force on 1 October 2015 and omitted the requirement for a section 21(4)(a) Notice to require possession after the last day of a period of tenancy in compliance with section 21(4)(a). This however only applies to tenancies in England and tenancies which commenced on or after 1 October 2015 (which are new tenancies, not replacement tenancies). What if the section 21 notice expires and the tenant has paid rent up to a later date?