When a project goes over budget, the client doesn’t just get frustrated with the cost—they often blame the design.
That’s one reason architects increasingly prefer to work with builders who can tender and deliver under fixed lump sum contracts. It’s a way to protect not just your client’s budget—but your relationship with them.
At RSD Constructions, most of the projects we deliver are fixed price. We work with architectural and design teams from early stages through to delivery, helping lock in cost certainty before a single footings cage goes in the ground.
What is a Fixed Lump Sum Contract?
A fixed lump sum contract is an agreement where the builder commits to deliver the entire scope of work for a single, agreed-upon price. That price includes materials, labour, subcontractor costs, prelims, and builder margin.
Once signed, the builder takes on the risk of cost overruns. Variations are only triggered if the scope changes.
In contrast, a cost plus contract passes the actual cost of the build—including overruns—directly to the client, with the builder charging a margin (often 15–20%) on top.
Why Fixed Lump Sum Benefits Architects
1. Design confidence
When you know what the project is going to cost upfront, you can design with certainty—not guesswork. No more late-stage redesigns because a builder’s provisional sum blew out by $200K.
2. Less conflict, fewer compromises
Clients can feel secure approving bold or high-spec features when they’re not bracing for weekly price surprises. Fixed price allows you to include detail and quality, while still respecting budget.
3. A clearer tender process
Builders working under lump sum know they need to price properly from the start. That forces rigour during tender, reducing the risk of disputes or mistrust once construction begins.
4. Cleaner contract administration
Fixed lump sum contracts are easier to manage—especially when the architect is acting as superintendent. No requirement to validate monthly cost breakdowns, invoice by invoice.
Where Cost Plus Fails Your Clients
Cost plus contracts are sometimes pitched as flexible. But in practice, they often cause:
Blowouts when unknowns emerge (with no cap)
Delays while builders reprice scope changes
Frustration as clients are forced into on-the-fly decision-making
Loss of trust when a builder seems to benefit from inefficiency
In a worst-case scenario, you’ve designed a home that can’t be built for the price the client thought they were committing to—and it’s your name on the drawings.
Good Documentation = Good Pricing
Let’s be clear: a fixed lump sum contract is only as strong as the documentation that supports it. We don’t quote lump sum off DA sets or council plans alone.
At RSD Constructions, we’ll price to full construction documentation, including:
Final architectural drawings
Structural and civil engineering
Interior schedules
Finishes, joinery, and lighting plans
That’s how we provide real certainty—for you and your client.
We’re also happy to offer pre-tender input on constructability, value management, and pricing assumptions. It’s in everyone’s best interest to align design with build cost as early as possible.
Want a Builder Who Helps Protect Your Design and Your Client?
We’re builders who understand the importance of well-documented, well-detailed projects. We respect design intent and we price accordingly.
If you’re working on a project that needs early cost feedback, or you want a builder who can tender accurately to a fixed price—get in touch.
Let’s talk about how RSD Constructions can support your next design from sketch to site.
