the limitation is not square metres as such, but total length of pipe.
The length of pipe in a zone will be a function of the laying density (i.e. pipe centre to centre measurement in whichever configuration you choose – I favour flow and return spiral)
The recommended maximum pipe run is 100-110m
If you do 200mm spacing, you will need c.5m of pipe for each m2, so 20m2 will be 100m and so should be doable with 1 circuit.
However the spacing is a function of the heating requirements – total KW or BTU of energy for calculated as necessary to heat the space required, which is a calc your plumber should be able to provide based on the u value of the walls, ceilings/roof, and floor of the extension.
In a well insulated new extension you might be looking for say, 100W m2 and at 50deg water you might need 100mm pipe spacing to achieve that depending on the system installed (rails, staple or grids) in which case its 2 circuits not 1.