What will happen if I paid for a security deposit towards an apartment but later changed my mind and never signed any agreement?

It depends what the deposit was for. A security deposit normally is a deposit to compensate the landlord if you leave without paying all of the rent due or cause physical damage to the unit. It would be fully refundable if you do not move in. A payment to secure the rental of the unit is a payment to the landlord to hold the unit available for your rental at a later date. You are not entitled to return of that sum because the terms of the agreement have been fulfilled the landlord held the unit for you but you decided not to move in. By holding the unit for you, they had to turn away others who would rent the unit so lost rent in not being able to rent it out earlier than otherwise. You do not provide enough information for me to tell which type of payment it was.

Sue the landlord in small claims court for the return of the deposit.

No, the landlord has no grounds upon which to retain your security deposit if you never signed a lease agreement and never took possession of the apartment! The landlord should return the security deposit to you forthwith. I suggest you write the landlord and / or the agent for the landlord a letter demanding the return of the deposit in full.

Get your security deposit back in a residential scenario.

This is a difficult situation and kind of a grey area of the law.. Legally you did not sign a lease.. but if the monies you gave him were a deposit before singing a lease and he acted on reliance of that deposit by taking the rental off the market he was harmed by the delay in ultimately renting it and has a right to keep his damages.. I would need to know more to give you a more definite answer. but I would suggest he should be able to take 3 weeks of rent .. so how many weeks of rent was the deposit? (legally it is a maximum of 1 1/2 months rent or 6 weeks)