Electricity is the number one place tenants get overcharged. Government rate (via MEA/PEA) runs a few baht per unit; many buildings resell electricity to tenants through the landlord's private meter at a marked-up flat rate — ask what rate you'll be billed at before signing, and get it written into the contract.
Common area / "juristic person" fees are billed separately from rent in most condo buildings, usually calculated per square meter per month. This covers the pool, gym, lobby staff, and building maintenance — confirm whether it is already included in your quoted rent or billed on top.
Water is comparatively cheap and rarely worth negotiating over, but check whether it is metered per unit or split evenly across the floor — the latter is common in older buildings and can quietly cost more if you live alone.
Security deposits are typically 1–2 months rent plus 1 month paid in advance. Photograph the unit condition (and any existing damage) on move-in day and keep the timestamped photos — this is the single most useful thing you can do to get your deposit back in full.
Lease length and early termination: most leases run 12 months. Read the early-termination clause closely — some contracts forfeit the entire deposit for breaking the lease early, others only forfeit one month. This is negotiable before signing, not after.
A short-term (month-to-month or 3–6 month) lease usually costs a premium of 10–20% over a 12-month lease — worth it if you are still deciding on a neighborhood, not worth it once you know you are staying.