The main qualification for a place to become holy or called as Dham is that the Lord or His pure devotee should have appeared or had pastimes there.
Several other things that constitute a Tirtha /Holy place/ Dham:
1. Devotees must have performed (or be performing) spiritual activities in the place, and the tirtha must be visited by sadhus, saintly persons. In fact, the Vedic scriptures state that a person who visits even the historically bona fide places of pilgrimage only to take bath is no better than a cow or an ass. Visiting a tirtha means associating with the saintly persons in attendance. Canakya Pandita warns that we should avoid a place devoid of saintly persons. And a place bereft of talk of Krsna, or God, and service to Him cannot claim holy place status.
2. By visiting a tirtha we should feel enlivened in our Krsna consciousness; the tirtha should carry that potency.
3. The chanting of the holy names must be present as a prominent feature of the tirtha. Concurrent with that should be deity worship. Srila Prabhupada told us that as he established the various deities around the world, he worried that his disciples would begin to feel the worship as a "burden in the neck." But if the deity worship is going on uninterrupted and the devotees in the area are taking shelter of the deity, then that place is holy.
4. Prabhupada defined a holy place as wherever the Srimad-Bhagavatam was being honored. That might be in a large temple or under a tree, and it may be in India or elsewhere, but wherever there is respectful and repeated reading of the Bhagavatam, that place becomes holy.