Can a contractor be held to a quote made from plans?

One of argument that says they can is the fact that the plans show the dimensions, and therefore they are a reliable reference point for quoting. One of the arguments against that position is that you can’t know the dimensions of something fully until it is built. The central issue underlying this is simply: to what extent can one rely on plans as a representation of the scope and extent of the work?

In my experience, plans are fairly unreliable and can only ever be a starting point for pricing. After all, if plans were the final word then there would be no variations, no changes at all, and no payment disputes. Here are some of the shortcomings of plans or drawings that explain why contractors often do not actually get to build what is on the plan:

Plans are often Temporary

A plan is given a revision number and in reality only reflects the works at a particular point in time. As we know, they are often revised, sometimes half a dozen times if not more. So where does this leave the quoting party who has provided as price for Revision B but is now asked to construct to Revision F? No matter how familiar you make yourself with the site and plans, if they change significantly so must your price.

Prone to error and omission

Where there are humans, there is human error, and so it is with plans. Often the plans have errors in them. Some of the scope may be missing. Some of the dimensions may be wrong. Elements may be in the wrong place. Specifications may be incomplete. The list goes on and on. There is no substitute for actually building something to realise the applicability of the plans. Of course, errors like this usually result in amended plans, but nothing changes the fact that the poor contractor had to price off the erroneous plans to start with. That is the point.

Additions and Variations

Often the client will simply want a whole section of particle item included that was never contemplated. A homeowner may decide to add a courtyard. A builder’s client may decide to change the roof design. This is just extra work and is a variation. In this instance, a totally new drawing will be issued and will need to be quoted, but sometimes the manner in which this new work interacts with the existing scope may change the cost to build that existing scope. Of course, the contractor could not have known that. The same applies to plans that have clouded areas at time of tender. These are areas that are not specified until later revisions. Only then can a final cost be confirmed.


For reasons that have never been able to pin down, two complementary trades working from the same drawing will find that their interrelated works do not match up. For example, one party may build some large sliding doors to be inserted into a void in a wall, but when it comes to installing the doors in the void, they don’t fit. The reason is that often the drawings do not account for some vital bit of preparation or some other work that relates directly to that product, or some other item that must be installed first. In this example, perhaps that brand of door requires some special associated elements before installation. Of course, that is the responsibility of the door supplier and installer to pick up but the point is that often such details are not on the plan.

Another aspect of plan buildability is that plans sometimes do not account for the simultaneous installation of multiple trades. That is, the various spaces and voids in the plans cannot accommodate all the services required in that area. This is only picked up when this is being built. Now some trades will need to rework their services at extra cost. Again, none of this would have been obvious from the quoted plans.

Don’t account for lack of trade coordination

Often, plans do not indicate a correct or ‘best practice’ order in which elements are to be installed. So even though all contractors are building to their drawings, they still cannot accommodate their works alongside each other. For example, mechanical services often bang into installed plumbing pipes, which then get in the way of the fire sprinkler pipes, and this is further complicated because the ducting has gotten in the way. With the advent of 3D planning, one can now predict the best order to install, but this is often cast aside by the desperate need to meet the programme. Sadly, many subcontractors’ costs are driven up in rework in order to meet the programme. The bottom line is that they ultimately do not build what their plans require them to build. They actually have to build endless workarounds.

So whether you are quoting on a schedule or a lump sum, it would serve all parties well to realise that you often don’t know what you’ve actually got to build until you build it. And it follows that you actually don’t know what it’s going to cost until you build it either.