Start with the business problem, not a feature list. Which people will use this, with what frequency, and what does the process look like without it? An experienced team who understands the goal will suggest a cheaper route to it; a team that receives only the requirements as given can only price exactly what you asked for.
Set out the scope as short scenarios: what the user does and what the system does in response. Equally important, list what is out of scope. build an mvp explicit list of exclusions saves more disagreement later than the rest of the brief combined. Mark too which items are decided and which are still under discussion — the difference changes the price, and pretending everything is fixed only hurts you.
Write down the hard constraints. These include the platforms and custom software development services involved, the data you already hold and its condition, security and compliance rules, traffic expectations, supported browsers or which is better vue or angular devices and infrastructure that is already decided. If a deadline is real, say why: a team will often cut the right scope to protect it, but only if they know it exists.
Write down what done means for each item. Clear acceptance criteria do not require special syntax: a short list setting out what a user should be able to do is sufficient. This one section compresses the review at the end considerably and closes off most late-stage disagreement.
One last thing, state what you want software development companies in europe the response. Require an itemised estimate, a written list of assumptions, the main risks and a range rather than a single figure. Take a broad range as a signal about the brief: it tells you exactly which requirement is unclear. Then rewrite that part and ask for a new estimate — the revised figure tends to be much more reliable.