Validating controls based on other Controls

Hi,
Firstly I have many controls, mainly radio buttons, that have required validators and a validation summary.
 
I have some radio buttons which contain property types, e.g. house, flat, maisonette etc. Then I have another set of radio buttons with floor level (Top, Middle, Ground). How can I make floor level mandatory only when flat or maisonette is selected and still be shown in the validation summary?
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