The first offer may not include them
Does this offer address the value of my remaining property, or only the land taken?
The initial offer often centers on the land being acquired. Severance damages ask a different question: what happened to the value of everything that is left.
If the offer does not say how the remaining property was valued, that is worth asking about before you respond.
An independent appraiser can review both parts, the land taken and the property that remains.