How many lodgers are allowed before HMO registration becomes necessary?

In the scenario of you + couple + 1, it would meet the prescribed description of an HMO, so Additional Licencing would apply. Conditions are not as onerous as for (larger) Mandatory HMOs though.
Solid old doors are usually fine, except perhaps for kitchen-hall (high risk room onto escape route). Mains wired smoke alarms (1 each floor) will be expected (same if you let the whole house to a family), but mostly they will simply check bedroom sizes on a floorplan you provide against standards - checking it's not overcrowded. That's the one thing to check yourself before applying.

Thanks Simon, your thoughts are rather as I feared so I'd have to give two lodgers notice, if I wanted the couple, or go down the route of an HMO. I've had mains-wired smoke alarms fitted as a condition of the loft conversion so I could keep the original doors and, after part of the kitchen ceiling collapsed,have fitted fire resistant plasterboard on that ceiling.

The Housing Act 2004 schedule 14 (6) states the maximum number of lodgers living in a property, before it is classed as an HMO, is two.
If three or more lodgers live in a property, in addition to the landlord and his or her family, the property will be defined as an HMO.
It doesn’t matter if the lodgers are related to each other or not – the rule is around forming two or more separate households.

As it's owner occupier, pretty sure it's different rules. Check housing act 2004 s,255 on legislation.gov.uk

There are all sorts of loopholes

Reply to the comment left by Judith Wordsworth at 21/07/2023 - 14:45
if the lodgers are related they are one household

Reply to the comment left by Judith Wordsworth at 21/07/2023 - 14:45
Yes, I found that out Judith when the Council confirmed that I could only have two lodgers before being registered. Looking, as you mention, at the Housing Act 2004 s.258 as quoted by Shelter it defines a single household as a family, for example a couple (whether married or not and including same sex couples) or persons related to one another. On this basis, I could let to a couple plus a single lodger [and even a student cousin!].
Having let these rooms for a good many years I have found that it's usually a good atmosphere with more than one lodger - and it keeps a pensioner on his toes!

It is number of persons, not number of households, that matters in defining an HMO for Additional Licencing --unless ALL occupants are related to one another.

Reply to the comment left by Simon F at 21/07/2023 - 20:35
In Manchester, 5 occupants are needed before licensing. Generally, flats with 4 people seem fine, although they can be HMOs. Not all relations are counted. We had two couples in a flat where the women were cousins, but, although they were related, this does not count as 2 households,

As a regular contributor to this site I find it quite concerning so many different interpretations of what constitutes the correct procedures to apply in running HMOs. This is type of development seems to be expanding hugely particularly in areas with very large homes that families no longer want. Clearly not a straight forward system!

The rules about how many people will form a household are different if they are lodgers who live with the property owner. As has been stated above, the number is two - which is set by The Licensing and Management of Houses in Multiple Occupation and Other Houses (Miscellaneous Provisions) (England) Regulations 2006 section 6(2) which provides the number omitted from the Housing Act 2004 Schedule 14 section (6)(1)(c).

It says 'persons' rather than 'households' so that probably means that if you rent to three sisters as lodgers that would make an HMO. However Schedule 14 section (6)(1)(b) says that any member of the household of the property owner won't count towards an HMO, so if you take in three of your own sisters, that will be OK!

Note that the fact that a property is an HMO does not mean that it is a licensable HMO. The mandatory licensing limit is five persons in two households but this can be changed by individual councils.

You may be interested in this website which has free legal and other information for lodger landlords https://www.lodgerlandlord.co.uk/