Start with what the number is for, because the purpose decides how precise you need to be. If you're planning neither to sell nor to borrow against the property soon, there may be no pressing reason to nail down an exact figure at all. If you do need one, understand why this particular situation resists the easy tools. A merged parcel is rarely worth the simple sum of its parts. Joining an adjoining lot to your existing parcel can change buildability, use, and desirability, so the combined value can land above or below the two values added together. Automated online estimators handle merged, unusual parcels poorly, because they key off standard records and comparable sales that don't map onto your situation. Two other wrinkles. A tax assessor's land value usually differs from what the land would fetch in a true market sale, so don't lean on the assessment as your answer. And land gets valued very locally, by people who know what parcels like yours actually trade for in that specific area. The practical path is a local real estate agent who knows land in your market. A good one will often give you an informed opinion of value at no cost, and for combined parcels that judgment beats any quick online tool. If the number has to hold up formally, for a loan or a legal matter, that's when a full appraisal earns its fee.