In most areas, staircase baluster spacing codes require that balusters be spaced no more than 4 inches apart, ensuring a 4-inch sphere cannot pass through any gap.

7.5 inches space is acceptable here

Kid safety is the reason why most new building codes require spacing of 4” or less. In my area, if you change the railing you need to conform with the 4” or less code. Even if 7.5” is allowed for your project, prioritize the 4” spacing for safety rather than the aesthetics of 7.5”.

If you go with 7.5”, consider adding plexiglass on the inside up to rail height.

The narrower spacing looks better.

to my mind a kid should not be unsupervised in the staircase and gates should be used in every enrty point because even if you have narrow spacing still the kid can fall down the steps which possibly will be worse.

narrower will look better

Thanks all for your opinions. What about my second more technical question:

How can I install the slats in order to have a clean look. Almost invisible connections with the ceiling and the wall.

Imo narrower looks much better, the wider spacing looks like you skimped on the job.

If only on one side I think they can be really cool but on both it looks like a jail cell. I do like the narrow spacing the connections will need to be discussed with someone who understands what you want and what is possible . Is it possible you are not in NA since you say the spacing is to code where you live? I have seen so many staircases in Europe where they would never be allowed in NA and the kids do not seem to be injured anymore there than here. Of course kids need to be careful but common sense seems to be rare here in NA so we have codes to fit the bottom level of common sense .

How best to securely attach the slats will in part depend on the slat wood type, individual slat dimensions and spacing. The installed slats need to withstand the weight of someone leaning against them or falling onto them.

If you have access above the ceiling, you may be able to install nailing boards above that will give you a strong connection when you screw or nail down into the top of the slats.

"A kid should not be unsupervised" is true and also irrelevant. Kids will always end up unsupervised at some point. They are sneaky and fast, and sometimes the on-duty parent needs to stir a pot, use the bathroom, etc. Plan for all the safety needs because they WILL get to every dangerous place when you least expect it. (Also kids mostly just bounce down stairs ok but stair railings can be deadly.)

@Patricia Colwell Consulting yes indeed I am in Europe.

@Design Fan (NE z7a) I will be usued solid oak, most likely 6x4.3cm (2.3x1.2 in). The ceiling part where slat will be attached is concrete and the wall is mostly bring and for the last 10cm below the tread was empty space where we placed some layers of MDF waiting for the slats.

Code and design magazines be dammed ,anything between 4 and 9 inches is a death trap for small children. Infant and toddler heads are larger than their body so they can go feet first through an opening and break their neck or suffocate because their head can't pass through.

The ceiling part where slat will be attached is concrete and the wall is mostly bring and for the last 10cm below the tread was empty space where we placed some layers of MDF waiting for the slats.

The only part of that I understand is that the ceiling is concrete. The only way to attach slats will be to mechanically fasten a beam to the concrete and attach the slats to the beam. If seeing the beam bothers you then the only other choice I see is to drop the rest of the ceiling to be flush with the bottom of the beam.

The biggest impediment I see to pulling this off is the timber. Sourcing straight, flat material and keeping it straight and flat considering you have only two points of attachment may be next to impossible.

I don't know if there is an engineered wood product that you can substitute for sawn timber?? Maybe steel or aluminum rectangular tubing instead?