As a first step I would remove the first floor door and basement door to the stairs and live with it for a couple of months.

Then I would determine how much money you are willing to spend on construction to get what you want.

Then I would find the best local honest builder to give you an estimate of what the construction cost would be.

Then comparing the budget with the estimate I would evaluate the project.

Okay, I’m prepared to spend on this and open it up as much as possible. I would like to know what is possible first before contacting a builder.

You can remove everything that's not structural. The floor plan is not a structural plan, so at the least an experienced contractor is needed. But, really, remove the doors first and see if this much openness suits you.

We just altered a legal two-family plan to a single family, leaving out the dividing wall and door at the basement level. Now that the build is framed and drywalled, it's going back in next week. We don't need to hear our sons and their friends.

It looks possible construction wise by the drawings provided. Now contact the build to see if it is possible. Houses are not always build as the drawings show. The builder can do the necessary onsite investigation to determine if and how it can be done.

You are asking for a direct funnel of major noise and HVAC inequality.

The layout is not affected. But to be effective to transmit all the noise, the middle wall between flights needs removed. This can all be done with much more millwork and trim, and probably have to keep a post on this floor.

There is no basement door, that is to storage.

And when the kids are older, you will probably want to undo what you redid.

Expensive and will affect how efficiently your house will heat/cool.

"I would like to know what is possible first before contacting a builder." Mark Bischak made the point that homes are not always constructed per the plans. You will be well served to ask local contractors to take a look at the existing conditions and advise you regarding what is possible, what is practical, and what the likely cost will be to modify your home.

Why would you want to have another opening to a stairway, when you've had to gate off the one going upstairs?

With technology, you can keep eyes and ears on those in the basement…..until they ar big enough to outsmart you.

Pull up photos of "open switchback staircases" - that will provide you with many ideas of all the ways people have done this.

I have an open switchback staircase - however, there is a door to the basement off the landing on the way down to the basement (so while it looks open all the way up to the second floor + down to the basement, there still is a door that you can't see that closes off the bottom half of the staircase that's in the basement.

I have a very large picture window on the landing that heads up to the second story = lots of light pours into the open staircase. Your staircase looks very dark - maybe that's because you have room darkening drapes over a window on your landing - or maybe you don't have a window like I do.