Side door left unlocked in apartment block?

If the door is unlocked, people can come and go into the building as they please whether or not they live there.

If this is a purpose-built apartment block, there will be an owners management company (OMC) which is responsible for the maintenance and operation of the common areas (including the side door). Each owner of an apartment in the building will be a member of this company and if they have concerns they can pursue them with the company.

As (I presume) the tenant of an owner of an apartment, you would have to get your landlord to pursue the issue with the OMC; he's the member, not you. And you've already said that he's not bothered. You could talk to your neighbours and see if they feel as you do. If some of them are owner-occupiers, they can raise the matter at the OMC.

It's not illegal to leave a door unlocked. You're only entitled to the exclusive occupation of your apartment; he can let anyone he likes into the common areas.

It's a big security concern. I wouldn't be happy. Get on to your landlord again more forcibly. Dosen't sound like it is run by a management company.

Could there be implications for the insurance of the building I wonder?

its not illegal but its expected that the building (inc. common areas) is for the occupants and their guests, not accessible to the general public

Where is "everyone" going, and what makes them unsafe?

If you keep closing it, people are gonna start realising that it’s not gonna be available to them as an entrance. They’re gonna make other arrangements, either getting keys or using the main entrance.

Who is the door for, why does it exist? You keep saying you want it lock, but seem to be ignoring that it surely has a purpose.

Are you sure it is not a alternate escape pathway?

FYI the same rules don't apply. It's not a MUD. It's a single property being sublet. The Landlord is essentially the Owner's Company.

who uses it? Do you use it?

I'd be pretty confident that occupants and their guests make up the vast majority of the users, if not all of the users.

Yup, I do understand how fire exists work. I'm questioning if you do tbh. The OP's issue is that the door is left unlocked, not simply left ajar. It is illegal to lock a fire exit. If this were a required fire exit the Landlord legally could not lock it.

No such planning or building control exists. It's perfectly acceptable to use fire exits are normal access and egress. Simple example, the primary fire exit of most houses is the front door.

Where door are designated as emergency exists only, that is almost always an operational or design consideration.

Not closing the door is lazy and rude. That can be fixed with a simple piece of hardware. A door closer, it won't solve all your issues. But its a start.

So neighbour wants it unlocked so people go that way. And you want it locked, so people go the other way. I assume you see the issue, neither has any more right or say in which door should be used. Maybe the neighbour is a biggest pain that you for the landlord.

There's no obligation to maintain a locked door because it was locked when you moved in. Is there anything in your lease agreement about security? You could request that the door is fitted with a closer and that tenants are given keys as appropriate. This could reduce traffic and might be the best compromise between two options.

You said there was 20 flats. So I would assume that means there is multiple stories, multiple flats per level. As its a converted house, corridors may not be fire protected, so I would expect a second means of escape to be required. You said there was a fire escape adjacent, but it was locked. That alost certainly means that an escape is required and at least one of the doors needs to be unlocked and a means of egress.

The door has been opened by the landlord. People won't get keys cut, they'll break the lock or the door.

The OPs apartment is at the most convenient access point to the building. There's no way they are going to lock it now, it's the default entrance to the building now.

Incorrect on both counts tbh. Required exits from a building typically include main entrances that are accessible in both directions. You are confusing a

dedicated fire exit (egress only) with a require exit pathway.

And the OP's issue not just that the door is left open, its that it is unlocked. This was literally the first sentence in the thread;

I've no idea how you've interpreted the locking, of lack of, to not be the issue. When OP has specifically said that's the issue, repeatedly. People leaving it ajar is careless, but it's not what is causing it to be used.

How does closing it stop people from using it? It's kinda simple, the door is now unlocked, so people are using it. When the door is closed, which is presumably most of the time, people use it to come and go. They just walk up to it, and open it. The neighbours sound careless, but they are hardly so dumb that they can't navigates a closed and unlocked door.