Garage floor level

"The best way to raise your garage floor by 180mm is to either pour a concrete overlay or install a precast concrete flooring solution. Another option is to pour a new concrete slab or install wooden floor joists with a suitable flooring material on top. It is important to consult with a professional builder or engineer to determine the best solution for your specific needs."

Best way in my view would be to lay a damp proof membrane followed by insulation sheets 2400x1200x80mm two layers would give you 160mm then lay 22mm tongue an grooved moisture resistant chipboard flooring glued at all the joints this would give you a well insulated floor of 182mm

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You can use 6” by 2” joists, 22mm way rock then ply over the way rock