ᐅ Musty odor in the guest bathroom (newly built)

Hello dear forum members,

We have been living in our small newly built house for about a year now. From the very beginning, I noticed that the guest bathroom on the ground floor often has a musty smell. I already mentioned this to the construction manager and the plumber back then, but all I got were responses saying that it could not be related to the installation. They suggested I look for the source of the smell outside (probably a draft coming in from outside). In the meantime, I also opened the access shaft but could not detect any odor at all. So, I suspect the source is the toilet itself. The problem is that the smell does not always occur but happens quite frequently (usually every day, sometimes several times even when the room was aired out in between and everything smells fresh). Can someone advise me where I should look so I can specifically request the builder to investigate further? I cannot detect the often-discussed fluctuating water level in the trap, at least not when flushing the adjacent toilet.

Thank you very much!

tm

Are the installations used regularly? Issues like this often occur due to a dry trap or faulty traps.

What does the overall installation look like? Where is the vent for the wastewater system located? Is it far away? Has a pressure equalization valve possibly been installed in the guest bathroom? Do you have an easy way to look behind the pre-wall installation? Do you have a construction plan or detailed plan for the house installation (sanitary)?