How much clearance is provided when taking off over a 50 foot obstacle?
Calculated figures for take-off distance required (TODR) are to only just clear a 50-foot obstacle at that distance (with gear down), and figures provided by aerodromes for take-off distance available (TODA) are the measured distance to the obstacle, with no extra clearance added. 

If there's any uncertainty when they measure TODA, they'll give the most pessimistic value. The obstacle may be further away than the given TODA but no closer. Measuring distances is easy with modern equipment so it's likely to be an exact figure. However, it's rare for the first obstacle to be exactly 50 feet high. If the obstacle is (say) only 30 feet high, you get 20 feet of separation for free.

Be sure to apply the standard safety factor (e.g. 1.33) when comparing your calculated TODR with the published TODA, to ensure you get plenty of separation over the obstacle. As StephenS points out, if there are any significant obstacles in your take-off path, it's good practice to go for a best-angle climb until you overfly them.