The Smoke and Carbon Monoxide Alarm (England) Regulations are here.

 Landlords 247will not allow a new tenancy without a smoke alarm as standard on every floor. FACT.

• Install a smoke alarm on each floor of the premises on which there is a room used wholly or partly as living accommodation;
• Equip a carbon monoxide alarm in any room of the premises which is used wholly or partly as living accommodation and contains a solid fuel burning combustion appliance (e.g. a coal fire, wood burning stove);
• Check that each prescribed alarm is in proper working order on the day the tenancy begins if it is a new tenancy (this part only applies to “new” tenancies that start on and after the 1st of October 2015).

Read this interesting 

The Smoke and Carbon Monoxide Alarm (England) Regulations 2015: Q&A booklet

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/458550/Smoke_and_carbon_monoxide_alarms_-_landlords.pdf