You can't build anything on the easement. You might be able to get whoever has the easement to abandon it, but doubtful.
All depends on where the easement is , and how it is written .
I know of a property that has an easement above and below for a neighbors house's septic drainfields and so he has parking for guests . The owner of the large house bought the neighboring little house , put an easement on the little house and then sold it . Now the guy in the large house has lots of room , and the guy in the little house gets to pay the taxes on the property , maintenance , and gets no benefit
The answer to your question depends on the nature of the easement itself. @Wayne Brooks could be correct if the easement really precludes building on the surface, but it might not. You indicated this is a drainage easement, which can be fuzzy, meaning is may give the right to drainage over a portion or all or the site. You won't know until you get a copy of the easement and read it.
So do this: call your local title company or title rep, ask them to issue a Preliminary Title Report and ask them to include backup documents. They may call them by different names in your area, but the concept is the same, you want all documents that are recorded against the parcel of land, that's fundamental to all property.
Open up the easement and read it, it may be complicated, but in most cases you may be able to understand the nature of the easement. If the legal description is too complicated (generally most are written as "metes and bounds") ask the title company to plot the easement and send it to you. If it's a general easement over the entire property, you will likely see that for yourself when you read it.
If the drainage easement is in a specific area over your site, then you'll know you most likely will need to keep your buildings out of that area. Is there already a drainage channel there? If so, it will be somewhat more clear, and the drainage needs to stay in place. If it goes through the middle of the site or a part that you want to use, you might ask the easement holder (called the dominant tenement, which means they own the easement and control or dominate that area of the easement) if you can relocate the drainage in your new project. They may or may not agree, and you will likely pay for any drainage relocation.
If the easement is general and over the entire property, you might ask the easement holder if a drainage course or structure can be built to accommodate their need for drainage, then redraft the easement to include only that area of the new drainage course thereby freeing the rest of the site from the easement and allowing you to build on it. Again, you will pay for the drainage construction and all legal costs for amending the easement.
Note: I say you will pay, because that's what any sophisticated easement holder would ask (that's what I would ask) and they do this because they control the situation by owning the easement. You are asking their permission and approval to change things, they'll want to keep their costs low. You can always ask them if they'll share, but if you want them to agree, any incentive you can offer them to agree will help your case.
Hope that helps.
General note: Our company offers real estate advisory services, DM me if you need more sophisticated help.
We have an unbelievable number of easements on a lot of our properties - drainage, sewer, utility, right of ways. Lots of them. The only one that was a problem was when the sewer line wasn't actually in the easement - after we shifted the re-routed the sewer line and had to shift the placement of the house slightly we were good and ready to go.
I don't shy away from a piece of land because of some easements. It's just one more information to review during due diligence.
Talk to your lawyer or surveyor and they can tell you the exact details on the easement.
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