Just make the frame a little bigger than the concrete so the outer rim sits on posts into the ground.
It might be too big of a step up though, so consider building a step all along the three open sides to make access easier.
Not far oiff the way we went in the end. Me and a mate got it done in 3 days together.
Step 1 removed 49 flag stones, not fun.
Step 2, realised that it was not a concrete base, but can only be described as 20 year old ***** patio job. A crumbling pile of sand, broken bricks, tiles and any other general garbage they could find, held in a square shape by breze blocks on the outside.
Step 3. Excavated a trench around the edges and dug 30 cm deep squares to cement post brackets and set the posts.
Step 4 built the frame from Tanalised 3m long joist planks
Step 5 laid polyurephane sheeting over the crumbling pile of **** under the frame.
Step 6. The easy bit ....screwing the planks onto the frame, with 5mm spacing
Step 7 . Built the step, because of the slight slope oif the garden the eight of the deck was only too high on the side facing the house, so built a step.
Step 8. tried to clad the deck but ran out of deck boards, kind of finshed with tw 2.4m boards my neighbour had but will replace with full length ones soon.
Final step today, laid fresh topsoil and sowed new lawn seed on the surrounding area around the deck.
Pricing -
Frame and Joists - 10 planks at £13 each - £130
Deck Planks - 25 3.6 m £5.98 each half price at
wickes -£149.50
Screws 2 x packs of treated deck screws 6.99 each £13.98
Post brackets and bolts - 4 x £9.99 = £40
1 x 10 cm square fence post - £11.99
2 x sacks of postcrete £15.98
Tools £175 - Couldn't resist a Makita driver/drill, made seriously short work of the hundreds of screws that needed driving + all other necessary tools
all in £536.41
Labour - zero , me and me best mate!