Interventoría: why the one who builds cannot be the one who checks

The question almost always arrives with a note of embarrassment: "am I going to pay somebody to watch over somebody else I am also paying?". Put that way it sounds like mistrust, or like an expense that could be saved.

It is neither. Nobody finds their own mistakes with the same enthusiasm they bring to finding somebody else’s, especially when correcting them costs money and time. That passes no judgement on anyone’s honesty; that is how incentives work. Interventoría is the answer Colombian practice gives to that: independent technical supervision of the works, engaged by the client and answerable only to the client.

Colombian law recognises this, and in some cases makes it compulsory.

Three things that get confused

When the law requires it

There are two cases in which independence stopped being optional in Colombia, and both come from the same idea.

Independent technical supervision, above 2,000 m²

Buildings on a site that would allow more than 2,000 m² of built area must be subject to technical supervision independent of the builder. And that supervisor is the one who issues, under oath, the certificado técnico de ocupación (the technical certificate of occupancy), the document from which the ten-year warranty on new housing starts to run.

Independent review of the designs

Above that same threshold, the structural designs must be reviewed by a professional other than the designer and independent of them in employment terms, at the expense of whoever applies for the permit. The permits regulation repeats the point: the reviewer "cannot be the same professional who prepared them, nor may they have an employment or contractual relationship with that person".

Note what is being demanded: it is not enough for it to be a different person. They have to be independent in employment terms. Two engineers from the same office do not satisfy it. That level of detail only gets written down after somebody has already tried the opposite.

What an interventor does

• —Verifies that what is built matches what was designed, and that the materials are the ones specified rather than the ones that turned up.
• —Controls quantities. What is invoiced has to be there on site. That is where the largest gap between what is paid for and what is carried out usually appears.
• —Checks the programme and records delays as they happen, not when a claim is made.
• —Requires and files laboratory testing: concrete strength, compaction, quality of the reinforcement steel.
• —Approves or rejects interim payment certificates. This is the most important function and the one that confers real power.
• —Leaves a written record. Site diary, minutes and a dated photographic record.

The last one is the most underrated. An interventor with no authority over payments is an expensive spectator. If the builder gets paid the same with or without that approval, the interventoría is decorative.

What it costs

The Colombian reference is still the 1989 fee schedule (no longer binding, but the place almost every quotation comes from):

Put in context: 4% on a job is less than a single badly executed item usually costs once it has to be demolished and done again. Interventoría does not always pay for itself, but it pays for itself with one such item avoided.

Independence is the entire point

If you take one thing away from this article, let it be this: interventoría only works if whoever carries it out does not depend on whoever builds.

The three ways of breaking that, all of them common:

• —The builder "finds you" the interventor. However competent that person is, their next contract depends on that relationship, not on yours.
• —The interventor comes from the same office that designed the work or that is building it. This is exactly what the rule prohibits for the review of designs.
• —The interventor is paid against progress on site. If they are paid when the work advances, their incentive is for it to advance, not for it to stop when it ought to stop.

Is it worth it on a house?

Below 2,000 m² it is not compulsory, but the logic does not change with size. It is worth having when any of these apply:

• —You cannot be on site. You live in another city, or you have a job. Interventoría is your technical presence.
• —You did not choose the builder. They were recommended to you, or they came with the plot.
• —The contract is fixed price. There the builder’s incentive is to cut costs, and somebody has to verify that the cuts are not made in what cannot be seen.
• —The contract is cost-plus management. There the incentive runs the other way (the fee grows with the cost) and somebody is needed to validate the quantities.
• —There is structure involved. Extending, adding a storey, strengthening. What is done badly there does not show until it matters.

And when it probably is not worth it: a minor refurbishment that leaves the structure alone, with a maestro de obra you know (an experienced master builder who takes small jobs on directly), in a house where you are there every day. There the interventoría is a cost with no function.

What the paperwork is for

Interventoría reports look like red tape until the day a claim has to be made. Then they are the only evidence of what happened and when.

It matters especially on new housing: the builder is answerable for financial loss for ten years counted from the certificado técnico de ocupación. Ten years is a long time to argue from memory. With a site diary, minutes and dated photographs, it is a different conversation.

Interventoría does not make the works more expensive. It makes doing them badly more expensive, which is exactly what it is for.