Can I fence off part of my yard that is covered by a registered utility easement?

Often not without the utility's consent, or at least not in a way that blocks their access. Even a fence, which seems far less permanent than a building addition, can interfere with a utility's ability to enter the easement area for inspection, maintenance, or emergency repairs, which is exactly what the easement exists to protect. Some utilities will permit fencing with conditions, such as requiring a gate or removable section that preserves their access, while others may restrict fencing in the easement area more strictly.

The specific answer depends on the exact wording of the registered easement and the particular utility's policies, so this isn't something to assume either way based on general practice. A fence that seems harmless to you might still technically obstruct the access rights the easement was created to protect, and a utility discovering an obstruction during maintenance work may require you to remove or alter it at your own expense, sometimes without much notice.

Before fencing off any part of your yard that falls within a registered utility easement, check the easement's specific terms and contact the utility or rights-holder directly to confirm what's permitted, rather than assuming a fence is automatically fine just because it's not a permanent structure.