Fixed Price vs. Cost-Plus: How 78704 Builder Contracts Actually Work
Developer Intelligence · August 2026
The short answer: a fixed-price contract moves cost risk to the builder, who prices that risk into the number. A cost-plus contract keeps the risk with you and charges a lower fee for it. Neither one is cheaper by nature. The difference is who absorbs the surprises — and on a 78704 custom build, the surprises are largely a function of how complete your drawings were on the day you signed.
Most owners walking into a first builder meeting have a vague sense that fixed price is "safer" and cost-plus is "riskier." That framing is close enough to be useful and wrong enough to be expensive. What actually determines your outcome is the allowance schedule, the change-order language, and whether the design was finished before the contract was signed. Here's how each structure works and where each one breaks.
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Fixed One Number Builder carries cost risk and prices for it | Cost-Plus Actual + Fee Owner carries cost risk, sees every invoice | GMP The Middle Path Cost-plus with a ceiling | Allowances Where Budgets Break Under both structures, equally |
What Each Contract Actually Is
Fixed price — sometimes called stipulated sum or lump sum — means the builder commits to deliver the house described in the drawings and specifications for one agreed number. If lumber goes up, that's the builder's problem. If a subcontractor bid comes in high, that's the builder's problem. The builder knows this, so the number includes a contingency for exactly those outcomes. You do not see that contingency broken out, and if the project runs smoothly, the builder keeps it.
Cost-plus means you pay the actual documented cost of construction plus a builder fee, typically expressed as a percentage of cost. You see the subcontractor invoices, the material receipts and the labor. There is no hidden contingency because there is no need for one — the cost is whatever it turns out to be. A smooth project costs you less than it would have under fixed price. A rough one costs you more.
In this market, builder fees on cost-plus custom work commonly land somewhere in the mid-teens to low twenties as a percentage of construction cost, depending on the firm, the complexity and the level of service. Fixed-price numbers for the same house typically come in higher than the expected cost-plus total, because the builder is charging you for certainty.
The framing that actually helps: you are not choosing between a cheap contract and an expensive one. You are choosing whether to buy an insurance policy against cost overruns. Fixed price is that policy. Whether it's worth the premium depends on how likely the overruns were in the first place — which is a question about your drawings, not about the contract.
Where the Money and the Risk Actually Move
Factor | Fixed Price | Cost-Plus |
|---|---|---|
Cost overrun risk | Builder | Owner |
Cost transparency | Low — you see one number, not the build-up | High — you see every invoice |
Savings if it goes well | Builder keeps them | Owner keeps them |
Drawings needed at signing | Complete — the number is only as good as the set it prices | Can start with less, though incomplete drawings cost you either way |
Incentive on material selection | Builder gains from spending less within spec | Fee rises with cost — requires a fee cap or GMP to align |
Owner time required | Lower — less to review monthly | Higher — monthly draw review is real work |
Best fit | Finished drawings, defined scope, owner who wants certainty | Evolving design, complex site, engaged owner |
General characteristics of each structure as used in the central Austin custom market. Individual contracts vary substantially. Have any construction agreement reviewed by a Texas construction attorney before signing.
The Allowance Schedule Is Where Fixed-Price Budgets Break
Here is the thing owners consistently miss: a fixed-price contract is not fixed for anything covered by an allowance.
An allowance is a placeholder dollar figure for a category that hasn't been selected yet — plumbing fixtures, appliances, lighting, tile, countertops, cabinetry hardware, landscaping. The contract says "$60,000 allowance for appliances." You select $95,000 of appliances. You owe the difference, plus in many contracts the builder's fee on the difference. Your fixed price just moved.
This is not a builder trick. It's an unavoidable consequence of signing a contract before you've picked your finishes. But allowances are where the majority of "my fixed-price build went over budget" stories actually come from, and the defense is straightforward: before signing, take every allowance line and go price real products you would genuinely choose. Not the cheapest option that satisfies the category — the one you actually want. If the allowance is $40,000 low across the schedule, you now know your real number, and you can negotiate the allowances up front instead of absorbing them as overruns in month eleven.
A builder who resists that exercise is telling you something. A builder who walks you through it, category by category, before contract is telling you something better.
Change Orders and the Language That Governs Them
Every custom build has change orders. The question is what they cost you and how fast you find out.
Read for these four things specifically. Does a change order require written approval before work proceeds? It should. Verbal approvals on a job site are how disputes start. Does the builder's fee apply to change orders? Usually yes, and that's reasonable, but you should know it. Is there a stated turnaround for pricing a change? Without one, a pending change can idle a trade for two weeks and cost you schedule you never get back. What happens to the schedule? A change order that adds four weeks should say so in writing, so nobody is surprised at month sixteen.
The change orders that hurt are rarely the ones you initiate. They're the ones triggered by conditions nobody knew about — rock where the excavation was supposed to be soil, a utility in an unexpected location, a structural condition revealed during demolition. On sloped 78704 lots with mature trees, those are common enough that the contract's language about concealed conditions deserves as much attention as the price.
Have a builder proposal in hand and want a second read?
The Davis Agency has been on both sides of these contracts across Barton Hills, Zilker, Bouldin Creek and Travis Heights for nearly two decades. A read on the allowance schedule and the change-order terms — before you sign — tends to be worth considerably more than a read on the price.
The Guaranteed Maximum Price Middle Path
A GMP contract is cost-plus with a ceiling. You pay actual cost plus fee, but the builder guarantees the total will not exceed a stated maximum. If it comes in under, the savings are split according to whatever the contract says — sometimes entirely to the owner, sometimes shared.
GMP solves the main structural weakness of cost-plus, which is that a percentage fee gives the builder no financial reason to spend less. Under a straight cost-plus arrangement, a builder who saves you $80,000 has just reduced their own fee. Under GMP with shared savings, they've earned part of it.
The trade is that a builder pricing a GMP is carrying ceiling risk, so the ceiling gets set with room in it. A GMP that's 15 percent above the honest estimate isn't much of a guarantee. Ask how the maximum was derived and what contingency sits inside it.
Questions Worth Asking Before You Sign
Ask what the builder's fee actually covers — whether supervision, project management and warranty administration sit inside the fee or get billed as general conditions on top of it. Ask to see a sample monthly draw package from a recent project so you know what documentation you'll receive. Ask how retainage works and when the final payment releases. Ask what the warranty covers, for how long, and who answers the phone in year two.
And ask the question most owners skip: what were the last three change orders on your most recent 78704 project, and what caused them? A builder who answers that specifically and without defensiveness is showing you how they'll handle yours.
Frequently Asked Questions
Is cost-plus or fixed price cheaper for a custom home?
Neither is inherently cheaper. Fixed price includes a contingency the builder keeps if the project runs smoothly, so a well-executed project under cost-plus usually ends up costing less. A project with significant unforeseen conditions usually costs less under fixed price, because the builder absorbs the overrun. The structure determines who benefits from the outcome, not what the house costs to build.
What is a typical builder fee on a cost-plus contract in Austin?
Builder fees on custom cost-plus work in central Austin commonly fall in the mid-teens to low twenties as a percentage of construction cost. The range reflects real differences in what's included — some firms bill supervision and general conditions separately, which makes a lower headline fee less favorable than it appears. Compare what the fee covers, not just the percentage.
What is an allowance in a construction contract?
An allowance is a placeholder dollar amount for a category of work or materials not yet selected — appliances, plumbing fixtures, lighting, tile, landscaping. If your actual selections cost more than the allowance, you pay the difference, often with the builder's fee applied. Allowances are the most common reason a fixed-price contract does not produce a fixed final cost.
Should I have an attorney review my builder contract?
Yes. On a project representing seven figures, a construction attorney's review is inexpensive relative to what it protects. The provisions worth professional attention are the change-order process, concealed conditions, lien waivers, retainage, dispute resolution and warranty terms. This article is general information and is not legal advice.
Related Reading from The Davis Agency
→ Who's Actually Building in Barton Hills Right Now
→ HOME Phase 2 Math in 78704: When a Lot Split Pencils and When It Doesn't
→ New Construction Homes in 78704: Build or Buy Smart
→ What 78704 Luxury Sellers Need to Know About the Mid-Year 2026 Market
Working Through a Build Decision?
Whether you're comparing two builder proposals, evaluating a lot for a custom project, or trying to figure out whether building beats buying in your price range, the conversation is the right starting point. The Davis Agency works with builders across 78704 daily and can tell you what a proposal actually says.
Start the ConversationCall (512) 608-8811
Derrik Davis · Broker/Owner, The Davis Agency · CLHMS Certified · TREC License #558841 · Serving 78704 and the greater Austin luxury market since 2006. This article is general market commentary and is not legal advice. Have any construction contract reviewed by a qualified Texas construction attorney before signing.
