Ecologists recommend preserving a larger share of the old-growth forests in Norway, as half of the species have forests as their main habitat. However, progress has been slow and at the time of this study only 1.4 % of the productive forest area had been preserved. We investigate benefits and costs over a 50-year period of increasing this level to 2.8% (doubling), 4.5% ("ecologists' minimum") and 10% (goal suggested in public debate). The benefits are estimated based on a national contingent valuation (CV) survey of Norwegian households. Two independent measures of total costs are used; i) the actual compensation amounts paid to forest owners, and ii) results from a survey of forest owners' minimum willingness-to-accept compensation to preserve. Results show that social benefits outweigh costs of all three conservation plans by a large margin. The middle option of 4.5% has the highest net present value. The results are robust to a range of assumptions, including considerations of potential hypothetical bias in benefit estimates. The results of this cost-benefit analysis reflect the preferences of the general population, the authorities and the forest owners with respect to biodiversity and ecosystem services preservation, and supplement the expert opinion of ecologists.