Main rental contract
Tenant T rents an apartment from landlord L.
The monthly rental is 1'000.
T paid a deposit of 3 months' rental = 3'000.
This deposit is in a blocked account, in T's name, but neither T nor L can withdraw the money from this account until they both give permission for it to be released. The deposit stays there, and is the security for L, in case T damages the flat or leaves it changed or dirty. In that case, at the end of the contract between T and L, L's costs will be covered from that deposit, and T will get the rest released.
Sub-let contract
T then lets out to sub-tenant S. (Okay, I think you've understood about getting the landlord's permission.)
Let's say the apartment has 3 rooms (two bedrooms and one living-room).
The living-room, bathroom and kitchen will be shared equally. This means that each person will have space to put their own things in these rooms, kitchen cupboard space, shelves in the fridge, etc.
Let's say Bedroom 1 is double the size (in m2) of Bedroom 2.
If T lives in Bedroom 1 and S in Bedroom 2, then T should pay 2x the rent that S pays.
1'000 is divided by them according to the m2 of their respective rooms.
T (in big Bedroom 1) pays 667 and S (in small Bedroom 2) pays 333.
Responsibilities and Deposit
T, now as landlord of S, has all the same responsibilities and duties towards S as L has towards T. In the same way, T collects a deposit from S (and places it in blocked account in S's name), to cover any damages S may make while living there.
Extra costs
In addition to the 1'000, there will be other costs, and these are usually shared not in proportion to the room-size, but 50%-50% (if there are two persons). Such costs include
- electricity
- gas
- water (although this is often included in the monthly rental).
These should be specified in the sub-let contract, and S pays their 50% share to T, each month. Alternatively, the sub-let contract can agree on a flat fee for these.
In addition, the
- so-called "Nebenkosten": this includes the heating bill, which the B will charge an estimated amount each month, and will settle up with the real amount after the heating season is over.
In any event, paying the extra costs, just like paying the main rent to L, remains the full responsibility of T, even if S ever defaults.