I got curious... so I calculated the numbers based on the data I've got available... This is a VERY rough calculation... definitely not something I'd put fourth in any kind of scientific study... some rounding is present that could lead to a significant margin of error... but it should be enough to get a general idea of the numbers we are talking about here... By the way AC... There's no need to get ugly. We're just talking about silly numbers and math here.
Mean radius of the earth at sea-level: 6371 kilometers
Convert to meters: 6371 * 1000 = 6371000
Volume of the earth: (4/3) 3.14 * radius cubed ~= 1083206916845753600000 cubic meters
Volume of the earth with 20 meters added to radius = 1083217118167215900000 cubic meters
Difference: 10201321462300000 cubic meters
Subtract 30% (3060396438690000) to account for land area not covered by water
= 7,140,925,023,610,000 approximate total cubic meters of water required to raise sea-level by 20 meters.
If I've done my maths correctly, this equates to nearly TWO MILLION CUBIC MILES of water (1,713,200 cubic miles | 7,140,925 cubic kilometers)... and could easily surpass two million cubic miles if land inundation were taken into account.
Now... I did a little further looking and some - apparently credible - websites cite as much as 6-7 million cubic miles of ice caps, glaciers, permanent snow, etc... so it is plausible that my previous assertion that there isn't enough water on land to pull this off is incorrect. However - even if that's true - we'd have to lose a HUGE portion of it to melting... plus... I believe that 6-7 million figure includes ice caps and glaciers already in the water - which would not contribute to sea-level rise if they melted.
So... I'm just sayin... all the hand-wringing about sea-level rise is a bit overblown IMHO. Guess what! The Earth is a dynamic system. It changes over time. Get used to it! Chances are, if this EVER becomes a problem - you and I will have been dead far too long to give a rip about our beach house.