Is size everything?

1. ‘Official’/universal minimum agreement on size of a bedroom?
Answer: It’s rent a coffin size where I live. Less than 64 Sq ft. Packing two strangers into a garage split by a wafer thin stud wall with one subsection in permanent darkness is perfectly acceptable as long as it has a window. From what I have seen in several areas of the country, if you can only fit a single bed in the space and stand up, then his is perfectly OK. This is the reality and I have seen it in council inspected properties that were authorised both before and after construction.
2. Does a bedroom have to be upstairs?
Answer: No.
3. Do I need to specify if a bedroom is a double or single size?
Answer: Only if two people are going to share the same room as far as I am aware. All of my rooms have double beds in them but they are single rooms as far as I am concerned.
4. What about odd shaped rooms – loft rooms with eves etc?
Answer: There used to be some details regarding only head height plus some clearance was relevant but I have certainly been shown property where it was impossible to stand upright in the connecting corridor. It looks like the inside of a wardrobe too shallow to take a clothes hanger side on, counts as part of the floor space now.
Good luck with the social consequences of this.

One reference point is the HMO standards for England and Wales. This specifies the minimum areas for both a double and single room. It's not a "perfect" standard as there is some ambiguity around the defintions.

The minimum size of a single bedroom in England is 6.54m. A smaller room cannot be rented out.

Reply to the comment left by Peter G at 17/01/2020 - 22:59Only applies to HMOs.

There are bedroom rules and over crowding rules to consider. Plenty of information about both online.

It will partly depend on who rents the house - a family or a group of friends sharing. A family is regarded as one unit. 3 or 4 unrelated people make it an HMO, 5 or more make it a licensable HMO. The number of tenancy agreements doesn't alter the above but can affect the number of mortgage lenders willing to lend.
Once a tenancy has been granted how a family choose to use rooms is up to them as long as the house isn't deemed to be overcrowded.

The rules that apply to licensed HMOs are very clear in terms of size and ceiling height. Smaller HMOs are a bit less clear cut but a phone call to your local authority Private Sector Housing team should clarify.

It's unfortunate that Planning Permission is still being granted for some incredibly small rooms. I saw one in a show house recently that was 4.12m2! They tend to be displayed as a nursery or study and described as bedroom 3. Having a legal minimum of 6.51m2 for all new builds would be a real step forward.
Smaller rooms can work perfectly well especially for children (and I'm certainly not in favour of prohibiting the use of any existing bedrooms) but if we are going to have minimum space standards let's start with not granting planning permission for any more tiny rooms.

Reply to the comment left by Jo Westlake at 19/01/2020 - 12:47
"if we are going to have minimum space standards let's start with not granting planning permission for any more tiny rooms."
I totally agree. Having allowed this disastrous social mess to develop, the first thing to do is to acknowledge this an stop it getting any worse.

Thanks all - I will read all your advice.
I want to buy a three bed house and if one of theexisting rooms is 'big enough' then thinking of splitting into two. House will be a cash purchase and only rented to a family so no issues of BTL concerns/lender issues/HMO probs. In this scenario just wondered is there a min size room for the one I spit into two. I am NOT a landlord that rents chicken coups, but there are more people looking for 4 beds over three where I live. Clearly splitting a room wont need planing permission. Other option is converting an existing garage into downstairs bedroom but that will mean more money and permission - something I want to avoid.