Developer-neighbor asking to build wall drainage on my side lot

[Will Park]

Hi BP! I need some advice regarding my neighbor/developer who is building a large multi-unit adjacent to my lot. Their new construction is below mine on a sloping hillside, and they want to build a concrete wall along the property line. On their side of the wall there are plans to build a walkway. However, they are asking me permission to build a 12-in wide v-ditch drainage on my side of the property for water that flows down the hillside and prevent the wall from eroding. They say that the v-ditch is necessary to keep integrity of the wall. My ultimate plan is to sell my lot to a developer who will likely construct a multi-unit- the lot is approximately 10k square feet and zoned for multi-units.

The benefit is that they are paying the cost to build a new wall between the two properties, as there is no wall there currently.

However, my concerns are (1) will this affect the future value of my lot- as the v-ditch encroaches onto my property approximately 12 inches, effectively reducing the useable square-footage of the property? (2) if I sell the lot to a developer who wants to use the v-ditch area along the wall for something else ie: a walkway or shrubs along the wall, will they be able to remove the v-ditch?

Can anyone provide some insight into this situation? I appreciate your help!

Will