Open with the reason this software should exist, not your preferred technology. What kind of user will use the system, with what frequency, and what does the process look like without it? An estimator who understands the goal will suggest a cheaper route to it; a team that receives only a list of screens can only price exactly what you asked for.

Set out the scope as user stories or scenarios: who does what, and what happens next. Every bit as useful, write down what is out of scope. An explicit list of exclusions prevents more disagreement at delivery time than any other single page. Indicate as well which parts are firm and top python development companies which are still under discussion — honest teams price those differently, and hiding it only hurts you.

Write down the hard constraints. The list covers existing systems the software has to talk to, the data you have and where it lives, php consulting services compliance requirements, expected load, which devices matter and stacks you cannot change. Where a date is genuinely fixed, say why: a good team can often resequence the work to protect it, but not if the date is a secret.

Define what the word done means for each item. Clear acceptance criteria need not use special syntax: a short paragraph stating what must be true when the feature works is enough. This one section reduces acceptance testing dramatically and closes off the most common source of disputes.

One last thing, ask for a specific format. Ask for a task-level breakdown, the assumptions behind each number, whatever the team considers risky and a range rather than a single figure. Take a broad range as useful information rather than evasion: it tells you the part of the brief that needs work. At that point rewrite that part and ask for a new estimate — the second estimate will be the one worth planning around.