As budapest said, it really depends on how much you need the money, or indeed how hard you want that money to work for you.
On the one side, the costs of keeping it are not insignificant: you still have to get mortgage approval, which could cause problems with the Agent/builder if there are delays. You also have to do snagging, connect to the utilities, and furnish; all before finding and paying a letting agent, and then declaring for income tax.
If you're original intention was to flip it, then I'd nearly say you might be better off doing that anyway, especially if you have made some Cap Appreciation.
heres a useful link for live pricing (add maybe 15-20% for a new build, as these figures includes all types of dwelling for each district).
the key column is 'average price m² (EUR)'. this is the only figure that matters when selling or buying. What you paid per sq.m, and what you think it will realise now per sq.m.
also, the same site is a hungarian equivalent of daft.ie to compare prices of similar properties to your own