I want to make a new flight of stairs for my own renovation job. I'm pretty sure that I'm ok with all the regs etc and confident that I can build it. However, I'm unsure how to secure it in place at the bottom.

It will be a free standing straight run stair case (so there are no walls either side to fix it to). I can anchor the flight in at the top to a joist running across the landing above with the use of coach bolts (or similar), but I'm stuck at the bottom of the stairs. The floor it will be resting on at the bottom is made of 3/4" T & G floorboarding over 4"x 2" joists on dwarf walls which means there is a small 18" void under the floor.

I think I'm be right in saying that it wouldn't be secure enough just to screw the string down to the floorboarding.  Would the joists under the floorboarding be strong enough to anchor into or, is this not advisable either? What alterantives are there to secure the bottom of the stairs for maximum stabilty and most importantly, strength?

Any advice appreciated.

as there are no walls,you will need handrails fixed to your newel posts.
extend these below floor level and fix to your joists.
with the use of extra trimmers.
hope this helps
john

P.S.i have never fitted a stair  
so stand correct if i am wrong!!!

There are lots of regulations for the stairs themselves but here are a  few for the handrails/ballustrading on stairs;

1.      The handrail should be between 900mm and 1m high. This measurement is taken from a line running along the stair nosings (pitch line).
2.      A handrail should be provided at least on one side and on both sides if the staircase is wider than one metre.
3.      There is no requirement for handrails beside the bottom two steps of the stairway
4.      The balustrade protecting the landing should be at least 900mm high.
5.      The gap between the spindles must not be so wide as to allow a 100mm ball to pass through.
6.      The balustrade should be designed so a child cannot climb over it.
7.      There is a further requirement that the guarding should be able to resist a horizontal force of 0.36kN for each metre of length. This equates to very approximately 40kg per metre.