Eva - I don't know if this helps, but the word "tentative" in the phrase tentative subdivision map, does not mean tentative in laymens terms, ie. the map is tentative and subject to changes in lot size, number of lots, street size, greenbelt area, etc. Tentative in civil engineering terms mean the map is not final and will still be checked for mistakes and compliance many, many times. The map will go back and forth between the engineer and the county many times until the county is satisfied with the map and records it. At recordation, the map is final as opposed to tentative, mistakes and all. Yes, even the mistakes are final and if the improvements are made to the mistakes, and the land is monumented, the mistakes will probably hold. The point is, the county uses the word tentative to convey non-finality to cover their keister and allow changes, not to allow profound development changes.