Drains issue in an apartment - any advice, please?

[CatAndHisKit]

Our apartment is on top (2d floor) of a small-ish modern block, with windows on opposing sides but drainage all goes down the back of the building.

Been here 18months and the drain smell problem started about 6months ago but only occasional, now it''s persistent. Most days, maybe once/twice a day, but on some days nothing. The heating pipes (combi boiler) I think run along the drainage, and often it's agrravated when heating is on - it's never on for more than 10min at a time currently.

There aer two bathrooms and the narrow hall between them, kitchen and boiler on the side of the first bathroom, and my bathroom is wer the drain goes out to the back o the building. The smell is worst in my bathroom and now also often in the hall between bathrooms.

So usually it's afetr washing machine cycle (we only use an hour cycle) but never at once, it takes a few hours, also if there is a lot of washing dishes in the kitchen but with something stronger (i.e. fish) involved. Or if the first bathroom is used more than usual. Strangely sometimes there is no smell afetr a washing cycle.

The smell in my bathroom is like damp cabbage sort of thing if milder or stronger type more like a rubbish bin! It can be very strong. The quickest way to get rid is to run shower or tap over basin in my bathroom for five minutes or so, but if it comes on at night it can last maybe half hour without anyone dealing with it. Also afetr being away it can be there until water is run a while - I also now use bleach and use more |Harpic down the toilet than usual.

Used bleach and also used Mr Muscle drain unbolcker in both bathrooms, baths and basins (but only used twice at most). Maybe I need to use it in the kitchen too? I've also recently used odour neutralising liquid in washing machine - it helped wit smell after the washing cycle, but not totally and haven't helped with kitchen/bathroom smells - I don't know if its he same drain.

Is there anything else to do - or pay a plumber to put stronger chemicals as it seems like the draining is just very slow and stupidly designed for everything to drain via my bathroom instead of separate drains to two halves of the flat. Will it even work? I don't really want to pay for a plumber visit if there is another way or they will juats say it cant be done?
Im also worried there might be a crack in the pipe and there is some stagnation somewhere. It's only a 12y/o. building though.

Thanks in advance for any advice! Sorry it's a long post.

More accurately it did start much earlier than 6 months ago but it was on and off maybe once in couple of weeks, though I can't remember precisely, mostly after being away.

@CatAndHisKit Do you know where the soil pipe is in relation to your apartment and others? It may be, without being too descriptive, poo in the soil pipe or the pipes to the soil pipe. If the contents of the pipe are not moved along by water, they will begin to smell, especially in hotter temp (you mention your heating being on). It clears when water is flushed along the pipe (when you run a tap for 5 mins).

It sometimes happen if the pipework is not used very often or has been used less than normal for a while.

I am not a plumber but experienced the exact same issue when renovating our house, we were not living there, a builder must have used the newly installed toilet, flushed but there was not enough water in the system to move everything along! The house was then left for a few days and the smell was not nice! It cleared by running the shower for 5 minutes.

Also to add, one of us cooks a lot (and washes dishes) and I'm sure it's contributing but they are not going to change that! Not spicy or fried food. Also the draining in sink / basins/bathrooms are fine. Occasionally slow in basins in bathrooms but instantly improved by Mr Muscle.

It maybe fat is getting into the drains this will smell and also slow or block them. I suggest you get a plumber or drain specialist to look, preferably with a drain camera.

Thank you! Isn't it really expensive to use a camera? Would you say better to ask a plumber or the drain person - and who would be more expensive?

Plughole are usually the place where sewer smells vent as they are the first open bit of the system. It's often an indication that the sewer outside is starting to block up although you would expect your downstairs neighbours to start to notice the problem first as well as neighbours in the next building in the street.

Ask around your neighbours. If they have problems too then call out the utility company who deals with sewerage services in your area.