She Is Becoming Mobile

Is the basement door right off the main rooms, or do you have to go through a hallway? You could block off the entire hall if it's not on the main room... Oor, you could get one of those baby corrals (looks like a bunch of interlocking baby gates) to set up a safe zone where you can stick her when you go downstairs. Then you know she isn't getting into anything when you aren't able to immediately supervise her. :)

Also... I would closely supervise her, and 'teach' her how to back down the stairs safely. That way, if she ever DOES get access to the stairs, she will know what to do instead of crawling and falling facefirst.

I got a baby corral, and every time I know she will be out of my sight for more than a few seconds she goes into it. She has toys to play with in there, and it has really helped her with her walking! lol. She loves to cruise around it holding on to the edges. (I have been using it with my DD since she started scooting.)

We installed a baby gate that attaches the wall. We do not have doors to either upstairs nor downstairs. Best thing we every did. I would recommend the babygate.

Our "side" door leads down to the basement and to the outside. We have a 1/2 door, meaning that the top half usually stays open, and the bottom half is latched on the basement side, so that the little ones cannot get down, but we don't have to close the entire door and can see and communicate. When my girls were about 4 we had to move the latch down so that they couldnt' reach it. Now that we are expecting again, I don't see us changing the way we use that door at all.

There are door closers you could attach. They bring the door closed automatically and are not too expensive. Like what you would have on a screen door, and that businesses use on outside doors.

we have the same issue with out basement, and worse it's a sliding pocket door, so it's a pain to close with an arm full of whatever. Thankfully it's off the kitchen so I just gated off the kitchen and thus blocked one set of stairs to the second floor, the kitchen, a bathroom and the basement. Even with the gate though we have to make sure it is closed, although after years of doing daycare from my home my older kids are now really used to closing it so it won't be a problem when the 5 month old starts moving.

I think you must install a baby gate. There is no way to guarantee that the door will always be closed or even latched when it is closed. My daughter was able to install a baby gate without taking the door off. I also wouldn't depend on it shutting on it's own. Good to have it automatic, I guess, but look at it to be sure it closed and latched. I've not seen one like this. If it's way more than one that you open and shut I'd go for the manual one.

Babies crawl fast and can be at the door before you get it shut. Also she will probably be upset that you've shut the door on her. She can't see you and with the gate, even if she can't see you, she can hear you and won't have the totally cut off feeling.