Who Should Actually Write the Spec? Decide With “Domain Rule Density” — and Price All Three Options

A 45-person medical device distributor budgets NT$1,600,000 for an inventory and approval system. The vendor asks for a requirements spec; the client's PM locks himself in a meeting room for three weeks and produces 22 pages of Word. Week 9 blows up: one order can ship in two batches but must be invoiced once — the warehouse lead has worked that way for 11 years and never wrote it down. Rework: 6 working days, NT$96,000, launch pushed back three weeks. Not a communication failure; spec ownership was never negotiated.

Four Myths Worth Breaking First

Myth 1: the spec is the vendor's job and belongs in the quote. The quote covers turning a spec into a system, not extracting eleven years of tacit rules from your head. In a free spec phase the vendor guesses, and the client still pays for the rework.

Myth 2: thicker specs are safer. Twenty-two pages with no acceptance criteria is worse than six where every line is testable. ISO/IEC/IEEE 29148:2018 decomposes a requirement into condition, subject, action, object and constraint — the point is verifiability.

Myth 3: it's faster to change as you go. Agile does not mean no spec. The GOV.UK Service Manual explicitly says you do not build during discovery — you understand the problem.

Myth 4: once signed, the spec is frozen. A spec is a baseline, not a seal; what the contract should fix is how changes are priced.

The Framework: Domain Rule Density × Documentation Muscle

DRD = the number of business rules that exist only inside your company and cannot be looked up externally. Every sentence like “unless…” or “only department X can…” counts as one.

Documentation muscle: in the past 12 months, did anyone produce a 3+ page process document that is still in use? Written once and never read counts as weak.

TSC = client hours × NT$450 + vendor spec hours × NT$2,000 + rework from missed rules + cost of blocked days

Rework coefficients use medians: a rule found mid-development costs about 9 hours to retrofit, about 32 hours after launch.

The Same Project, Three Ways, Priced Out

Using the distributor above (DRD 24, weak muscle):

The option that looks free is the most expensive: the sprint costs NT$180,000 up front and still lands lowest, saving NT$99,000 and 15 working days.

Three Companies, Three Different Answers

• 12-person e-commerce brand (DRD ~6, weak muscle): rules are industry-standard; vendor-written at NT$45,000–70,000 is enough.
• 45-person medical device distributor (DRD 24, weak muscle): exceptions live inside long-tenured staff; a paid sprint at NT$150,000–220,000 is a premium.
• 200-person restaurant chain (DRD 30+, strong muscle, PMO): co-author — client writes 40% (rules), vendor 60% (behaviour), NT$120,000–180,000; full outsourcing here is waste.

Hidden Costs of the Client Writing the Spec

• PM opportunity cost: 120 hours = three weeks with the day job stalled; at NT$60,000/month plus employer costs the loaded rate is about NT$450 (MOL wage survey).
• Cross-department interviews: warehouse, accounting, sales, support at 2–4 hours each — 10–20 hours total.
• Spec review meetings: 3 × 6 people × 2 hours = 36 person-hours, about NT$16,200.
• Vocabulary translation loss: the client writes “automatic,” the vendor reads “scheduled batch job”; each misread burns 4–9 hours.
• Missing acceptance criteria and version chaos: the Word file reaches version 7 while development builds from version 4; disputes drag 5–12 working days.
• Rules that surface after launch: a 3.5× rework coefficient plus the trust cost with your customers.

A 10-Dimension Scorecard for the Spec Phase

Score each 1–5, 50 total, before a line of code exists.

Below 30, do not develop. 31–40, close gaps before signing. 41+, proceed to contract.

How ScriptWalker Fits — and the Honest Caveats

Standalone paid spec sprints (Advisory model, 2–3 weeks, NT$120,000–220,000), copyright with the client, usable for competing quotes and not tied to us building it; if we build, 50% of the fee is credited against development.

Three things that do not favour us. Vendor-written specs carry structural bias toward the stack we know, so every line must be read back by your frontline staff. A sprint is guaranteed revenue for the agency, so you are entitled to a deliverables list before the second instalment. And we routinely recommend cutting features you asked for.

Do not hire us if: you only want leverage over your incumbent vendor; you will not let frontline staff be interviewed; all the rules live in one departing, uncooperative employee's head; or the core is machine, PLC or production-line integration.

Playbook: 14 Days Before Kick-off Through Day 30

• Day −14 to −8 | Exception inventory: department heads submit their “unless” lists, target 20+; the one task a client cannot outsource.
• Day −7 to −1 | Materials: three months of real data, the five most complex documents, current Excel and its formulas.
• Week 1 | Field observation: sit with the warehouse lead and the accountant half a day each; watch, do not listen.
• Week 2 | Draft plus reverse read-aloud: the vendor writes, frontline staff read each line back as “customer returns half — what do I click?” What they cannot narrate is not written.
• Week 3 | Freeze: acceptance criteria, out-of-scope list and CR rate card delivered together.
• Day 30 | Review: more than 5 new rules in the first month means the sprint was incomplete.

Decision Checklist

• Can list 15+ rules unique to how your company operates
• A process document from the past 12 months is still in use
• Willing to give frontline operators 2+ hours of interview time
• Can supply three months of data and the five most complex documents
• Every requirement has a testable acceptance condition
• Have an explicit “not building this” list
• Know who owns the spec copyright
• Hold a change-request rate card
• Someone can sign off and freeze the spec
• Accept that the spec phase is billed separately
• Understand a rule retrofitted after launch costs 3× or more

Eight or more: go into development. Five or fewer: run a sprint before quoting.

FAQ

Do I always have to pay someone to write the spec?

No — not if DRD is under 15 or usable documentation exists. When DRD is high and muscle is weak, the rework on a “free” spec runs 1.5–2× the spec fee.

Can I take a paid spec to other vendors for quotes?

Yes, and asking is a good way to judge a vendor. Put copyright and unrestricted use in writing.

How detailed does a spec need to be?

The bar: a frontline employee can read it aloud as operating steps. Every “unless” must map to a defined system behaviour.

A rule was missed mid-build — whose cost is it?

Check the client's “unless” list. In the list and not implemented is a bug; not in the list is a change request priced by the CR rate card.

Will a spec sprint delay launch?

No. The 2–3 weeks come back multiplied during development — a net saving of 15 working days in the case above.

Next Step

Start with an exception inventory. Send us your “unless” list for a free 30 minutes: we calculate your DRD on the spot, map it against the matrix, and tell you who should write this spec and roughly what it costs. Nothing to sign.

• Email: [email protected]
• Phone: 0916-224-047
• LINE: @ufv9089p