@Alejandro Hinostroza @Scott McIntosh is right. Sorry there seems to be some incorrect info elsewhere in this thread so listen to Scott. The substantially improve clause applies to the cost/assessed value of the building not the land. So if you pay $100k for an OZ prop, $60k for the building, $40k for the land, you have to invest an additional $60k within 30 months to develop the property. There's some wiggle room to decrease these $60k, however I suggest staying in this threshold and within the spirit of the law.
If there is no building the sunstantial improvement clause does not apply, however the substantintially ALL clause requiring 90% of the fund's assets to be allocated to an OZ prop/business still applies. So in the above scenario, you pay $40k total just for land and your yotal fund aize is $100k, you still need to invest $50k for a total of $90k deployed to develop the OZ property within compliance.
These are some of the tricky nuances, however don't let it dissuade you. Here in Pittsburgh, CBRE is doing both land and development deals in OZs so they are definitely commercially viable.