Flat Fee vs. Hourly vs. Cost-Plus: Which Actually Makes You Money
Short answer: hourly protects you, flat fee scales you, cost-plus funds you — and a firm past its first few hires should be running at least two of the three at once.
The question isn't which model is best. It's which model fits which phase of which project, and whether your scope document is strong enough to survive the one you picked.
The three models, honestly compared
Typical 2026 range — $150–$450/hr
Flat fee: Set from your own project history
Cost-plus: 15–40% markup
You get paid for — Time spent
Flat fee: Outcome delivered
Cost-plus: Goods procured
Rewards — Thoroughness
Flat fee: Efficiency
Cost-plus: Volume
Punishes — Efficiency
Flat fee: Weak scope
Cost-plus: Client buying direct
Client sees — Every hour
Flat fee: One number
Cost-plus: Sometimes nothing
Cash flow — Steady, lagging
Flat fee: Front-loadable
Cost-plus: Lumpy, late
Best for — Undefined scope
Flat fee: Defined scope
Cost-plus: Procurement-heavy work
Hourly: the safest and the most limiting
Hourly is honest. Scope changes, you bill the change. Nobody eats the difference.
It's also the model most likely to keep your firm exactly the size it is. Two structural reasons:
It penalizes competence. A designer who solves a layout in two hours instead of eight bills a quarter as much. You are paid for inefficiency and taxed for expertise, which is the opposite of what should happen as you get better.
It caps revenue at capacity. Your ceiling is billable hours times rate. Raising the ceiling means hiring more people or raising rates — and raising an hourly rate is the single most scrutinized number in any fee conversation, because the client can multiply it.
Where hourly is genuinely right: early concepting before scope exists, consultations, add-services after a flat-fee scope closes, and any client whose decision-making pattern you don't trust yet.
Flat fee: where the money is, and where it disappears
Flat fee is how a design firm gets paid for outcomes instead of hours. Get faster, earn more. That's the correct incentive and it's why most firms should be moving toward it.
It's also where most firms bleed, and the cause is almost never the number.
Flat-fee margin dies of scope creep, not underpricing. The fee was fine in week one. Then came the third revision to a room that was signed off, a two-month client delay that fragmented the schedule, and a "quick look" at the guest bath. None of it was billed. All of it was work.
Which means the flat fee is only as good as the document underneath it. At minimum, that document should state:
Revision limits. How many rounds per phase, and what a further round costs.
A defined deliverable list. What is in scope, item by item — and a short, explicit list of what is not.
Client response windows. What happens to the schedule and the fee when approval takes six weeks.
A change-order mechanism. A named process, a stated rate, and a form. Not a conversation.
A termination clause. What you're owed if the project stops at each phase.
Firms resist writing this down because it feels adversarial. It isn't. Every one of these clauses exists to prevent the conversation where you feel taken advantage of and the client feels ambushed.
Cost-plus: the quiet engine, and the quiet risk
Markup on furnishings — 15–40%, most often 20–25% — is often the largest single margin line in a residential firm, and the least examined.
Two failure modes:
Invisible dependence. Firms that price design fees too low and let procurement margin cover the gap have built a business that only works if the client buys through them. The day a client says "we'll source it ourselves," the project goes underwater and there's nothing to renegotiate.
Administrative drag. Procurement is real work — sourcing, ordering, tracking, freight claims, damaged goods, returns. Below about 15%, that work costs more to run than it earns. Firms holding a 10% markup out of discomfort are usually paying for the privilege of doing it.
The fix is separation. Price the design so it's profitable standing alone. Treat procurement margin as a second business line with its own economics, and know what each one earns.
How to choose — a decision path
Is the scope genuinely definable right now? No → Hourly, for this phase only, with a written plan to convert. Yes → Continue.
Will you be sourcing and managing FF&E? Yes → Flat design fee + cost-plus on procurement. Most established residential studios land here. No → Flat fee, with a change-order mechanism that actually gets used.
Is this a ground-up build or gut renovation with a large construction budget? Yes → Consider percentage-of-construction for the architectural coordination, plus flat fee for interiors.
Are you consistently profitable but exhausted? That's not a fee-model problem. That's a scope or capacity problem, and changing models won't touch it.
The hybrid most established firms land on
After enough projects, the majority of healthy firms we see converge on roughly the same structure:
A paid discovery or concept phase — flat, modest, and non-refundable. It qualifies the client and gets you paid for the work you used to give away in the proposal.
A flat design fee for schematic through documentation — the bulk of the fee, front-loaded, billed on a phase schedule.
Cost-plus on procurement — priced so the design fee would still be profitable without it.
Hourly for construction administration and anything out of scope — with a stated rate the client saw before signing.
It's not the simplest structure to explain. It is the one that survives the projects that go sideways, which is every project eventually.
Related reading
Frequently asked questions
Which fee model is most profitable for interior designers? Flat fee plus cost-plus procurement, for firms with defined scope and reliable processes. Hourly is the safest and the least scalable.
How do I move existing clients from hourly to flat fee? Don't convert mid-project. Use the next project, or the next phase, and frame it as budget certainty for them — which it genuinely is.
What's a reasonable number of revision rounds? Two per phase is the common standard, with additional rounds billed hourly at a stated rate. The specific number matters far less than having written one down.
Should I charge for the initial consultation? Yes, in nearly every case. A paid consultation filters out shoppers, and firms that switch to charging almost never switch back.
Is percentage-of-construction ever better than flat fee? On large new builds where scope will genuinely evolve with the budget, yes. On defined-scope renovations, flat fee is usually cleaner and less contentious.
19th & Co advises founder-led architecture and interior design firms on fees, hiring, operations, and business development. Founded by Christine Woodward — M.Arch, fifteen years in practice — the firm has worked inside more than 500 design firms, delivered over 3,000 coaching sessions, and advised more than 1,000 designers and firm leaders one-on-one.
If you're not sure your fee structure is the one your firm should be running, you can book a strategy call on my calendar here.
Last updated: August 21, 2026
