ᐅ Raising a terrace—how to do it? Perhaps using adjustable pedestal supports?

Hello dear experts,

We want to install porcelain tiles (Unicom Starker Debris) on our terrace, sized 60x120cm (24x47 inches). The manufacturer offers options with increased slip resistance in both 10mm (0.4 inches) and 20mm (0.8 inches) thicknesses.
The terrace needs to be raised by about 15cm (6 inches) compared to the current level. In addition, we plan to extend approximately 1m (3.3 feet) beyond the existing footprint into the current lawn area.
So far, the terrace consists of large sandstone slabs lying on sand/gravel. I don’t know the exact construction or depth, but there have been no cracks or other issues for the last 20 years.

At the moment, we have two quotes.
Contractor 1 wants to tear out the old terrace and build a completely new structure with a concrete base slab. For a very high cost.
Contractor 2 wants to pour concrete on top of the old terrace (plus of course an extension into the lawn). Still for a high cost.
In both cases, the tiles would be installed into the concrete (or a screed layer).

I would prefer an option with less ground disturbance and lower costs.

Could the thick 20mm (0.8 inches) tiles be installed on pedestals?
It is clear that the current lawn area will require a foundation, but would a strip footing be sufficient in that case?
What other options do you see?

2 cm (0.8 inch) thick slabs of this size without adhesive are viewed very critically by many. I researched extensively and asked around beforehand, and most people said that they should be glued.

We were also very uncertain and last year laid slabs measuring 90 x 45 x 2 cm (35 x 18 x 0.8 inches) on a bed of gravel. The joints are very narrow and filled with swept sand.

Below that is compacted crushed stone, with concrete curb stones set all around.

So far, we are very satisfied except for a few beginner mistakes (2 slabs are loose, the front edge is slightly raised, which causes algae growth over the winter and makes cleaning a bit more difficult). However, those issues could be fixed if we put in the effort.

Regarding pedestal supports with slabs of this size… I would be somewhat concerned that the slabs might break under unfortunate load conditions.

Large-format tiles are also currently popular for outdoor use.

After researching, my personal conclusion was:
Thin tiles (up to 2 cm (0.8 inches)) laid on gravel tend to shift or wobble sooner or later; if using tiles, it’s better to lay them on a solid foundation – although there is a potential risk of frost damage or joint deterioration.
Pedestal systems also require a water-draining layer underneath.

We ultimately decided on natural stone (local graywacke from Lindlar) (100 x 50 x 4 cm (39 x 20 x 1.6 inches)), with a flamed surface, laid on gravel. These slabs weigh about 60 kg (132 lbs) and sit firmly and securely (and can be lifted again). It is slightly more expensive (but not by much), yet simpler and cheaper to install.
Very comfortable underfoot, only minimal algae growth in spring, which can be scrubbed off with some hot water and swept away when dry. In summer, it does not get too hot.

Our terrace is also raised (visible height between 20 and 50 cm (8 to 20 inches)), shaped like a U on the south side around the house. The retaining wall, however, was very elaborate:
Strip foundation, formwork blocks filled with concrete, facing bricks matching the house in front, with a projecting natural stone coping (with rain groove) on top. Inside, in front of the coping, a concealed Aco channel drain directs standing water off the terrace slope.

Simpler options: planter blocks used as edge support (or formwork blocks).

My parents recently had 75x75x2 mm (3x3x0.08 inches) joists installed on pedestal supports. I don’t like it. Besides the strange noise when walking on it, it’s practically impossible for none of the panels to wobble.