Building Regulations regarding open tread/riser stairs

[FresnoBob]

Hi folks,

Our staircase is in our living room (and there's no easy way of relocating it), and the living room itself isn't particularly massive. The current staircase is quite overpowering (as well as being ugly generally). I'm considering whether replacing it with something like this would be feasible....

or something like the one in the centre of this picture....

However, I'm totally baffled by building regulations, mostly the situation regarding fire safety. Our current staircase is woooden with a plasterboard underside, which I thought was necessary under building regulations. Are the two staircases above compliant, or would something like a sprinkler system or similar likely be necessary?

 

[freddiemercurystwin]

You only need plasterboard if there's a cupboard or room beneath it and fire separation is required. Presumably you don't have a cupboard beneath as you're looking at these kind of alternatives?

The two examples would comply although only if a 100mmØ sphere could not pass through between the treads or balustrade. This is why in the second example rods have been fitted between the treads.