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I’ve got some basic carpentry skills and some used joists from a deck I want to use to build storage shelves under our house. I need help working out how to attach a timber frame to the base, which is mostly large uneven solid sandstone the house sits on. I need to work out how to fix and level the posts when the ‘floor’ is natural rock. Thanks
Afternoon @NadineC
Now that reminds me of my last place!
I happened to have built a set of long shelving on slopping paved ground. It was freestanding and solid.
Looking at your area (love under the house areas, its always "free" space!)
-On the left I would remove whatever rocks you can then level it tho on a slope type deal. Pave it if you have available spare pavers as it makes using the area so much nicer. (I would just use the dirt base under the pavers as they are not going anywhere soon)
-On the right I would find three or four points going up towards the rear wall from the front where you could put in timber stands.
-Connect the stands together using your joists and bolts. Then if you have access to some board use that for the base of each shelf.
Green = leveled ground as a slope with pavers if you have them otherwise just dirt.
Red = Timber posts resting on the odd shaped sandstone you refer to. They are not connected, just standing on it.
Blue = cross beams bolted to the posts (There should also be some solid rails that run along the top of these crossbeams to the next posts)
White = Boards to cover the rails between each stand so you can put your stff on them.
This way you dont need to excavate too much, its easy to change once done and can be built using recyled stuff. It was the first thing I built before any renoing I ever did 
Dave
Hi @NadineC what a great amount of under-house storage space. Just to throw another hat in the ring of ideas and one I have done for myself and is exactly the same project I'm soon to make for a mate. It's simply to use heavy duty stayed brackets secured directly to the wall without any need to build a timber frame. And would only require very little excavation underneath. Stayed brackets if your haven't used them before will not buckle under their rated loads.
Heavy duty stayed brackets range in sizes, I would go with 600 x 300 brackets fixed to the bricks (not the mortar course) using 50mm Ramset Universal anchors that each have a 32kg load rating in to brick.These brackets would be going anywhere.
Without knowing your height requirements, you appear to be able to have at least 2 rows of shelving. I would start with the far end as this is your lowest point.. Dig a channel deep and wide enough to fit the bracket to the wall and use a hammer drill to make the fastener holes. Then carry that level along the brick course to each bracket position.
You could start with 4 brackets per level and easily add more at a later stage depending on the type of eventual storage load. @JacobZ suggestion of pine decking, will provide a cost effective solution as would 1800mm yellow tongue flooring which is what will be using. But you could rip to a narrower width depending on your bracket selection. Having a shallower top shelf makes it easier to access items at the back of a lower shelf, if you can afford the space loss. The shelving would be simply fixed with 20mm 10Ga wood screws from the underside through the holes already in the brackets.
For the walk path, once cleared you could start with crushed rock mixed with cement then lightly watered in which will create a fairly solid floor as is. But would be an ideal base to lay those pavers @Dave-1 suggested. I also liked his idea to put some form of anti-head banging on the floor joists above.
Regards, Nailbag
