Decent Guide to Billing Fixed-Fee Design Projects by Progress

Fixed-fee projects sound simple until invoice time. The contract says one number, the team tracks time another way, the client expects a clean progress bill, and someone has to translate all of that into something accurate, readable, and defensible.

This guide is for firms that bill by phase, milestone, percentage complete, or professional judgment. The goal is not to overcomplicate the process. It is to make fixed-fee billing feel calm, repeatable, and easy to explain.

Three Things To Do

Treat each phase like its own billing unit

Break the project into phases that match how the client understands the work: schematic design, design development, construction documents, interiors, administration, or whatever your agreement uses.

Keep progress billing separate from timekeeping

Time is useful context, but it should not automatically decide the invoice amount on a fixed-fee phase. Use time entries to understand effort and margin. Use progress billing to communicate contractual value.

Make the invoice easy to read

Show the phase, total fee, previously billed amount, current amount, and remaining balance. If there are reimbursables, taxes, credits, or hourly add-ons, separate them clearly.

Three Mistakes Not To Make

Do not bury fixed-fee progress inside hourly detail

Hourly logs can be helpful attachments, but they should not make the invoice look like the project is being billed hourly if the contract is fixed fee.

Do not lose track of prior invoices

Fixed-fee billing depends on billing history. If last month is not reflected correctly, this month can easily overbill, underbill, or misstate the remaining balance.

Do not handle credits manually at the last minute

Initial payments, retainers, or prepayments should be applied intentionally and visibly. Manual spreadsheet edits right before sending are where mistakes creep in.

A calm way forward

Good fixed-fee billing is not about adding more administrative process. It is about having one clear place to prepare the invoice, review the logic, apply the right credits or rules, and send something the client can understand.

Decent Billing was built for exactly this kind of design-firm workflow: fixed-fee phases, hourly add-ons, Harvest time data, prepayment credits, taxes, expenses, and clean invoice preparation.