Smallest possible window for ventilation in utility room

That is understandable so far, only the side door will not quite be in the same price range as the front door. I think most don’t have that!? That’s why the question about a minimum.

Now seriously. Front doors usually have only one handle, mostly made of steel, for closing; opening is done by turning the key. You can lock yourself out. That’s why our back door has a real door handle to press, since we prefer to use this door, partly because the shoes are there; it’s almost always open during the day, sometimes at night too if we forget to lock it. We basically cannot lock ourselves out. Of course, it’s cheaper than the front one, but it has the same lock with the mushroom heads. It’s less well insulated, the utility room is unheated, no matter. It has less design, but otherwise it has no disadvantages.

That makes sense so far, only the side door probably won’t be in the same price range as the front door. I guess that’s true for most people!? That’s why the question about a minimum.

That’s why they are also a popular way for burglars. Most people still think of a proper resistance class for the front door, but then save money or simply forget that the basement door or garage can also be an entrance to the house. In the end, the question is whether you want to feel safe or not and what kind of protection the door needs for you. Insurance or not.

That makes sense so far, only the side door won't be quite in the same price range as the front door. I think very few people have that! That's why the question about a minimum.

The higher price range of a front door mainly comes from material, design, etc.
You need a proper frame, multi-point locking, etc., simply good protection against break-ins using screwdrivers or crowbars. That is also available for less money, since the side entrance door doesn't have to be a design marvel.

That is understandable so far, only the side door will not quite be in the same price range as the front door. I think very few have that!? That's why the question about a minimum.

It doesn't matter. If it had the same (good) feature as the front door, then a window would just be pried open.