TL;DR Disclaimer: I found out about the mechanic that I'm about to explain via this video ([Project Zomboid's Building Spawn Mechanic (for zombies)](https://www.youtube.com/watch?v=UOQaZ0PflZs) by Jiffydude) and the screenshot is also from that video, all credits to him. If you don't want to read about when and why zombies (don't spawn), feel free to watch that video.

Part 1:

Recently I noticed several players each day posting about situations where they were surprised on why they got attacked immediately after entering a house and couldn't hear any zombie activity beforehand. With this post I would like to take some time to explain the system behind why this is the case and how that can lead to many situations that seem unfair to many people, myself included before I knew about this.

  
In vanilla PZ, every house is technically empty until two conditions are met. This can most easily be seen via the screenshot I added, where Jiffydude placed some useful tiles around a door to show how close you have to be to a building for it to spawn zombies inside. But that's not all, because you ALSO have to have vision into that building at the same time. So:

\- Looking into a building fom further away than shown in the screenshot: no zombies

\- Standing directly in front of a door without having gained vision into the building: no zombies.

This has lead to many occasions of people being immediately attacked by zombies, notoriously in places like portable toilets and the little service buildings on the side of several roads in the game. Since there's no windows and you therefore can't look into the building, if the game decides to spawn a zombie in there after opening the door it will be right in front of you and come at you right away.

It does not matter at all which kind of building it is, this 2-part-rule applies to every single one of them. So no matter if you've been sneaking around and stealthily killing or running around the block mowing down everything with firearms: if you didn't get close enough to the buildings AND had vision at the same time, no zombies will have spawned in them and could always surprise you later on when you might not expect it anymore because you "cleared the neighbourhood".

Part 2:

Taking into consideration how that system actually works, I think it's a valid question to ask yourself: do I like that system? Does it break my immersion? Would I prefer all buildings to get populated once I enter the cell that the buildings are in?

If your answer to the last question is Yes, then I can recommend you a mod that I've been using since b41, which also works fine in b42. It's called [Wake Them Up](https://steamcommunity.com/sharedfiles/filedetails/?id=2972287975) and all it does is remove the building spawn rule that I explained here. From my own experience (I would guess around 1.000 hours of playtime with that mod enabled) I can tell you a bit about how that will change your gameplay experience.

In my opinion the world you're in becomes more dangerous on one hand, but also more predictable on the other hand, which in turn makes it less dangerous again if you know how to handle it. I always use pinpoint hearing for my zomboids, so take that into consideration. 

\- There will be a lot more broken down doors and windows than you're currently used to, especially when yelling or using firearms / other loud things like sirens. This will be the case even more so if you have enabled "Environmental Attacks" in the sandbox options, since zombies will then be able to attack doors and windows without having seen the player beforehand.

(I recently read from a couple of people that they feel the world is "too pristine" or not damaged enough. If you're one of those people, you might really enjoy the change this mod brings.)  
\- Streets will be more populated as a result of this, also especially when "Environmental Attacks" is enabled.  
\- This can in theory lead to some dip in performance / fps, depending on the area you're in and how loud you are.  
\- Just driving through a neighbourhood can lead to the entire street being infested with zombies when you come back later because now they actually spawn and follow the noise, depending on your settings like hearing and memory.  
\- You should be surprised way less by zombies as a result of all of this. I had a chuckle earlier today when someone posted about the Louisville hospital being empty, with a later comment saying (THEY WERE ALL ON THE 3RD FLOOR). This can't happen anymore if you use this mod, as soon as you enter that cell all of the zombies will immediately spawn in, possibly over a thousand of them, depending on where you are. This can feel like a disadvantage, but it can also be used to your advantage, depending on playstyle.  
\- Single zombies will still be banging on doors forever because they can't damage them until you are close enough to them, but at least you will be way more likely to hear them. (Not 100% sure about that, I recently saw a video that showed in debug mode that single zombies can't damage doors at all, but the creator wasn't directly next to the zombie, so take this with a grain of salt.)

That's the most important stuff I can think of right now, possibly I will edit this later if more comes to mind. If any of my explanations weren't clear enough, you're having questions or further context, feel free to add to this topic. Also feel free to comment if you were aware of the "BS" mechanic or not - from what I read online most people don't seem to know about this.

Cheers and happy zomboiding!

Zombie Building Spawn ("BS") Guide