Tibetan plateau is 4,500 meters above sea Level and Mt Everest sits on top of this.
Measurements for mountains are measured from Sea Level to the top of the Mountain = Method 1...
Method 2 is to measure from the Base of the Mountain, and Method 3 is to measure from the Center of the Earth. Actually, if measured from the sea level, Mt. Everest is the highest point on earth. But several other mountains are sometimes claimed to be the "tallest mountains on earth".
E.g., Mauna Kea in Hawaii is tallest when measured from its base with height over 10,200 m (33,464.6 ft), but only attains 4,205 m (13,796 ft) above sea level.
By the same measure of base to summit, Denali, in Alaska, also known as Mount McKinley, is taller than Everest as well. But its height above sea level is only 6,190 m (20,308 ft). Denali sits atop a sloping plain with elevations from 300 to 900 m (980 to 2,950 ft), yielding a height above base in the range of 5,300 to 5,900 m (17,400 to 19,400 ft).
In addition, the summit of Chimborazo in Ecuador is 2,168 m (7,113 ft) farther from earth's centre (6,384.4 km (3,967.1 mi)) than that of Everest (6,382.3 km (3,965.8 mi)), because the earth bulges at the equator. But its height above sea level is only 6,268 m (20,564.3 ft) while Mount Everest's if 8,848 m (29,028.9 ft).