How about this, made using Helical Ramp with sides?
I edited the ramp after drawing it, to make the bottom surface a copy of the top, then deleted the bit from there to the ground. However, it is still a parametric shape, and if you R click and edit it, you can change its parameters to suit you, then re-copy the top surface to the bottom, and delete what’s below it again.
Quarter helix stairs.skp (721.4 KB)
See what you think.
Not sure what version of SU you are using (please complete your profile) so in case it isn’t the latest, I’ve saved it as v2016.
PS. Because the winder treads have a 2" straight section at the front edge, the top helical side (which I made the same as the bottom one) won’t cover the inner edges of the treads there, unless you were to make a horizontal 2" stringer in front of them. I’ve compromised a bit here, but it illustrates the principle.
And you’ll need to trim a bit off the botttom helix where it goes below the floor.
You can’t get a same-slope join between straight stairs and helix unless you considerably flatten the slope of the stairs with more treads and a longer horizontal dimension for the straight flight.
PPS. CORRECTION: And I THOUGHT your winder treads weren’t exactly 30 degrees - I just copied the plan. The angle between the edges is 30 degrees, but neither edge is radial from the centre of the support pillar (quite possibly by design). That means I’ve rotated the treads slightly too far, since I used the back outside corner of the tread to define the rotation angle. So the back of the top tread of the winder isn’t at 90 degrees to the front of the bottom tread, which will cause constructional problems. Sorry. got that slightly wrong but haven’t time at the moment to fix it.