There is no universal rule that a basement counts for half. The 50% figure people repeat is a rough rule of thumb, and no appraiser is bound to it. How an appraiser values basement space comes down to three things: whether the space is finished, whether it has proper egress, and how comparable sales in that specific market treat below-grade square footage. An unfinished basement usually adds little to no value. A fully finished basement with a bathroom and legal egress can be valued much closer to above-grade space, though appraisers generally still separate below-grade from above-grade square footage rather than lumping them together. On the offer itself: if your comparative market analysis and your agent's CMA line up on value, that is a reasonable basis to write from. Just know the appraisal will ultimately lean on how local comps with similar basements actually sold, never on a blanket percentage. Ask your agent to pull comps that match the finish level, egress, and layout of this specific basement, because that is exactly what an appraiser will do. One caution: a year-old CMA is a starting point, and values move. Lean on the most recent closed sales you can find to support the number you are offering.