Main Contractor Vs Design And Build Contractor: Which Do You Need?

Developers often use “main contractor” and “design and build contractor” as if they mean the same thing. They overlap, but the difference matters, because it decides who is responsible for the design, who carries the risk when something goes wrong, and how much control you keep over the finished building.

The short answer

• Main contractor (traditional route): your architect and engineers complete the design, and the contractor builds what is drawn. You own the design risk.
• Design and build contractor: you set out your requirements, and the contractor takes responsibility for completing the design and building it. Most of the design risk passes to them.

In both cases the contractor is the main contractor on site. The difference is the contract and where design responsibility sits.

How the traditional route works

Your design team takes the scheme through planning and on to detailed technical design. The contractor prices a complete set of drawings and specifications, often with a bill of quantities, and builds to them.

Advantages

• Full control over design and specification
• Tenders are easy to compare, as every contractor prices the same information
• Suits complex, heritage or high-specification schemes

Disadvantages

• Slower, as design must be finished before tender
• Design errors and omissions become variations, which you pay for
• Split responsibility can lead to disputes about whether a defect is design or workmanship

How design and build works

You prepare Employer’s Requirements, setting out what the building must achieve. The contractor responds with Contractor’s Proposals and a price, then completes the design and builds. Often the developer’s architect takes the scheme through planning and is then novated to the contractor to finish the technical design.

Advantages

• One point of responsibility for design and construction
• Greater price certainty once the contract is signed
• Design and construction can overlap, so work can start sooner
• The contractor’s build knowledge feeds into the design, which can reduce cost

Disadvantages

• Less control over detail not written into the Employer’s Requirements
• Changes after contract can be expensive
• Tenders are harder to compare if proposals differ

Which should you choose?

Ask yourself four questions:

1. How much does cost certainty matter? If your funder needs a fixed price, design and build usually gives more certainty.
2. How complex is the building? Listed buildings, conservation areas and unusual structures often suit the traditional route.
3. How much time do you have? Design and build allows an earlier start on site.
4. How strong is your own team? Experienced developers with an in-house technical team may prefer to keep design control.

For many residential schemes of up to a few dozen homes, design and build with a novated architect is a common middle ground: you shape the design through planning, then pass responsibility to a contractor who builds it.

Getting the Employer’s Requirements right

On design and build, the Employer’s Requirements are your main protection. Be specific about the things that matter to your buyers and your funder: space standards, finishes, energy performance, warranty requirements and any lender conditions. Anything left vague will be interpreted by the contractor.

A note on modern methods of construction

Off-site systems such as timber frame, structural insulated panels and light gauge steel are more often delivered on a design and build basis, because the system supplier’s design needs to be integrated early. If you plan to use one, check that it is BOPAS-accredited, as lenders and mortgage providers will ask.

Final thoughts

Neither route is better in every case. The right choice depends on the scheme, your appetite for risk and how much design control you want to keep. Our guide on how to choose a design and build contractor in London covers selection in more detail, and you can see how Kisiel Group works as a main and design and build contractor.