Seoul rental market is very active and the realtor system is pretty much different from the western one.
Due to a different realtor system and market situation, realtors should contact the house owner every time to check the availability of the apartment whenever he/she has a customer.
Landlords normally have multiple realtors to find a new tenant for leasing their apartments.
So, it is hard for realtors to hold any apartment for their customers with commitment before they come and see the apartment in person and putting down some money to secure it when they found one they like to take it.
Landlords, of course, don't like to leave their apartment empty too long.
Chances are you will see empty apartments when you look around apartments in the market.
And most landlords reluctant to wait over 10~15 days after signing on contract even if you putting down 10~20% of your security deposit to secure it.
For most buildings which is over 6~7 story, not all the buildings though, every unit has its own different owner. So different owners ask different prices and different types of rental payment, from No-rent-deposit-only rental to low-deposit-high-rent. But the level of the deposit totally depends on the owner of the apartment. From one month rent deposit to several million or even a couple of billion won deposit.
When the security deposit is not as high as around 25 million won, it can be secured safe by Korean government law, only if you can prove that you are the tenant of the apartment by reporting to any government authority and suggesting the contract which shows the address of the apartment, rental period, names and stamps or signatures of both parties on it. (소액임차자보호법 Low-deposit tenant protection law )
When the security deposit is higher than 30~50 million won, they normally register their security deposit to government office to have a government stamp on the contract paper with the date they visited the government office on it. When the registered date is earlier than any other date bank mortgage was done, then it has precedence over the bank to take the deposit money back.
So, when you make a contract, you should ask your realtor to check the government paper which shows the address of the property and size, mortgage situations and ownership of the property.
If the total amount of the mortgages of the landlord plus your security deposit is over 60% or 70% of the market price of the property, you might not be able to take all the money back.
And one more thing to take into consider is that the total amount of the deposits of all the tenants who are living in the same building owned by the landlord. And those are not noted on the government paper but it also should be calculated together with bank mortgages of the building. So, when you rent with a high deposit, you should ask your realtor to check about this as well.