How to write a demand letter

A demand letter is the cheapest step that often works. It states what happened, what you want, and by when, and it creates the record every later step relies on.

What a strong letter contains

Facts in date order, the specific sum or action you want, and a clear deadline. Attach the documents that prove each point rather than describing them. Say what you will do if the deadline passes, and only say it if you mean it. Leave out insult and legal posturing entirely: the audience may be the other side today, but it can be a judge later, and a calm letter reads far better in that room.

Send it so it counts

How you send it matters as much as what it says. Use a method that shows delivery, keep a copy of exactly what was sent, and note the date. Email with a read confirmation, registered post, or both. If the other side is a company, send it to the address on their official registration rather than a branch, and keep the receipt.

What to do when the deadline passes

Do not send a second warning with a new deadline; that teaches the other side the deadline is not real. Move to the step you said you would, whether that is a small claims filing or a complaint to a regulator. If they respond with a partial offer, get it in writing before accepting anything, and be clear whether accepting closes the whole matter or only part of it.

FAQ

Does a lawyer have to write it?
No. A clear letter you write yourself works. A lawyer's letterhead sometimes adds pressure, at a cost worth weighing against the sum.
How long a deadline is reasonable?
Long enough to be answered and short enough to be taken seriously. Say a date rather than a number of days.
They replied denying everything.
That is still useful. It fixes their version early, which is harder to change later if the facts say otherwise.
Can a demand letter hurt me?
Only if it overstates facts or threatens something improper. Stick to what you can prove and to steps you are entitled to take.