Does there exist a single word, either an adjective or a noun, that effectively describes an individual who habitually underestimates things—e.g., cost, time required, complexity?
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How about an optimist:
If you really and truly want to make do with just a single word alone, you are going to have to go with a word like a chronic underestimater or underestimator, a misjudger or even a low-baller.
So it seems that they are perpetually falling short of the mark in their subaccurate assessments. Given that, I think what you are really looking for is someone who is actually overestimating not underestimating something. They are overestimating their own abilities at getting things done.
When this is a chronic characteristic, what you have here is someone subject to the Dunning–Kruger effect:
Someone who is actually competent will learn from their previous mistakes, and make allowances for all this in the future. But those of lesser competence are also less competent at assessing their own competence, and so always think they will do more in less time — and do a better job at it, too.
So I suppose you could them a victim of the Dunning-Kruger effect, or just a Dunning-Krugerite for short. I know I’ve certainly used the term with people whom I know are certain to understand me, but it is not in common parlance.
This may not fit perfectly in all situations, but if you'll take an adjective, I'd say either overconfident, naïve, or arrogant, depending on the situation. These are all (slightly) derogatory/pejorative, to varying degrees.
This can be used to describe someone's decision-making process in a given instance ("We didn't consider that there might be copyright-issues to navigate before we could start... we were overconfident...") or of a person in general ("Jason is just always overconfident").
Of the words I'm suggesting in this answer, this one is the most general, and probably the least insulting, as it simply carries the meaning that someone isn't putting the clues together about how difficult something is.
This word carries no implication as to why someone is not seeing a challenge clearly. My other two suggestions have more of a flavor as to the reason that someone underestimates.
This is a less-direct way of saying that someone tends to underestimate a situation, but it still carries the implication you want, because of the general tendency of people with little experience to overcommit, and fail to recognize hidden pitfalls in advance.
"Naïve" can also be used to mean that someone is simply acting inexperienced; that is, someone with some level of experience is simply not recognizing certain factors which signal that a task might be a struggle.
For instance: an architect of several years' experience needs to negotiate with property-owners to secure a build-site. The architect can design the building just fine, but if they falsely believe that the negotiations will go over quickly and without any compromises, you might call this naïve.
Finally, we have:
This would be used if someone is overconfident but not naïve. That is, they should have sufficient experience to know better, and they generally understand that the task at hand would be hard for other people to accomplish, but not for them personally; ego clouds their judgement.
It's not uncommon for someone who thinks too much of themselves (or in some cases their team or equipment or some combination) to think very little of extrinsic challenges. Saying this of a person immediately implies they may be prone to fail at properly assessing challenges or adversaries because of their own ego.
This is probably the least-pleasant thing to say of someone – not only because it derides their personal character but also because arrogance is generally not something that is transient, unlike overconfidence (which can be strictly situational), or to a lesser degree naïveté (which can go away after a person has the right experiences to learn a given lesson).
Maybe you'd refer to that person as "Panglossian", or "a Dr. Pangloss", after the highly optimistic character from Voltaire's Candide.
Or you might say she was "Polyannaish" or "a Polyanna", after the eponymous character from Eleanor H. Porter's famous novel Polyanna.
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