Send the same one-paragraph brief to three agencies and you'll often get three numbers that don't even look like they're for the same project — one at £15k, one at £60k, one at £150k. That's not agencies being dishonest, it's the brief being too vague to price consistently. Here's what to document before you ask for quotes.

User roles

List every type of person who will use the system and what they need to do. "Admin and regular users" is not a role list. "Warehouse operator scans stock in and out; ops manager approves adjustments and runs reports; finance views reports read-only" is.

Core workflows

Walk through the 3–5 things the system absolutely must do, step by step, as if explaining it to someone who's never seen your business. Agencies estimate based on workflow complexity far more than feature lists — a "simple" approval step can hide five edge cases.

Must-have vs nice-to-have

Separate what the system cannot function without from what would be nice eventually. Agencies that quote against an undifferentiated wishlist either over-price defensively or under-price and hit scope creep later — neither helps you compare quotes fairly.

Integrations

Name the actual systems that need to connect — your accounting software, a specific CRM, a supplier's API — not "it should integrate with our other tools." Integration complexity varies enormously between vendors and is one of the biggest quote-to-quote swing factors.

Data volumes

Roughly how many records, how many users, how much growth expected in year one. A system built for 50 records behaves very differently under 500,000, and this affects both architecture and price.

Success criteria

What does "done" look like? What would make this project a failure even if every feature technically works? This surfaces requirements people forget to mention until it's expensive to add them.

Why vague briefs produce wild quote spreads

When a brief is thin, agencies fill the gaps with assumptions — and different agencies assume differently. One assumes you need full audit logging, another doesn't. One assumes three integrations, another assumes one. The quotes aren't measuring the same project anymore, which is why comparing them directly is close to meaningless.

This pairs directly with knowing what to ask an agency before hiring them — scope your side first, then use that conversation to pressure-test their answers.