Start with the reason this software development outsourcing uk should exist, not a feature list. Which people will use it day to day, with what frequency, and what does the process look like without it? A vendor who knows what you are trying to achieve can propose a cheaper route to it; one who only sees a feature list can only price the list as written.
Describe the scope as concrete flows: what the user does and what the system does in response. Just as important, list what is out of scope. An explicit exclusion list prevents more friction during acceptance than almost anything else in the document. Also mark which decisions are settled and which are still under discussion — the difference changes the price, and concealing the open questions helps no one.
Set out your constraints. This means existing systems the ongoing software support company has to talk to, the data you already hold and its condition, security and compliance rules, user volumes, target platforms and stacks you cannot change. If there is a hard date, say what depends on it: a good freelancers or dedicated team will often rearrange the plan to meet it, but not if the date is a secret.
Write down what completion means for the important items. Testable acceptance criteria need not use special syntax: a short list stating what must be true when the feature works is sufficient. This one section shortens the review at the end by a surprising margin and removes the usual argument at handover.
To close, smm services for startups state what you want in the response. Require an itemised estimate, a written list of assumptions, the risks the team sees and a range rather than a single figure. Read a wide range as a signal about the brief: it normally identifies where your description is thin. At that point clarify that area and ask for a new estimate — the second estimate will be far closer to reality.