Question
Hi
I want to build some steps into my grass and was wondering If you could give me some advice on the best durable way of doing this.
The height is approx 2ft and the length is approx 20ft with a depth of 8ft down to 2ft. I was wondering if I should build a frame and pour concrete also thinking of the best way to prevent subsidence.
I am going to finish them with a slab on top.

Answer
Preventing subsidence is all in the foundation preparation.  Why not just start with slabs of rock on a solid compacted base?  It is a less expensive way, and I think a more attractive and informal use of stone.  A general guide is here:

http://www.hgtv.com/landscaping/stone-stairs/index.html

Rock has a charm that concrete doesn't have, even concrete hidden under ledge stone.  It is a bit more labor-intensive, and trimming around it once the lawn grows back takes a bit more care with a string-trimmer, but I think the effect is more inviting, and natural angles and curves are easier to achieve than with concrete.

An awesome 揾ow to?is here, with all the extra technical points:

http://www.sustland.umn.edu/implement/flagstone.html

The slopes you've described are not that excessive (less than about 5 feet in fall) but if your heart is really set on concrete, here is two good video links to that type of stair system.  One finish won't require added rock for a 搑ock?finish using form-liners, and the other is for rock veneer on concrete:

http://www.youtube.com/watch?v=QuqryhFu9w8  

(for a textured concrete finish that can be stained a natural color, such as behr.com or a similar product that should be available in the UK)

http://www.youtube.com/watch?v=1P5mQhOyaCA  

(poured for later flagstone finish)

Concrete pouring is easy.  Concrete finishing takes time and effort to do well.  It can be done by the average person.  I抳e tried my hand at it.  It is a learned skill like any other.  For the novice, maybe just a pour and then add the stone on top.  It would be a doubling of the materials used, but if you are trying to match stone work elsewhere in your yard or on your house, it may be the better solution.

So in order of difficulty: direct rock steps, concrete with veneer, then textured, finished, and stained concrete.

Best of luck! ~Marc

PS: When you mentioned "finishing with a slab on top" I assumed the treads & risers - If you were referring to a concrete or rock slab patio at the base or foot of the stairs, the formwork might be slightly different than in the videos. ~M