Why is it so important?
Landlords don't have time to read 200 applications in detail. Your cover letter must convince in 30 seconds. It shows: Are you reliable? Do you fit the apartment? Can you pay the rent?
What needs to be included?
Brief introduction (name, job, currently living), why this apartment (neighborhood, size, features - be specific!), why you're a good tenant (income, non-smoker, no pets if relevant, quiet lifestyle). Maximum 150 words.
Tone matters
Professional but personal. No standard phrases. Show you've read the listing. Landlords are people - be likeable but not pushy. Desperation is a turn-off.
Customize for each apartment
Never copy-paste! Every apartment is different. Adapt at least the first paragraph. Mention specific details from the listing. This shows genuine interest.