Urbanization is an established trend worldwide. It generates negative consequences in urban areas threatening their sustainability and future. In this context, cities are utilizing their underground space at an accelerating rate. However, usually underground alternatives are only considered as a last resort. At the same time, the underground space is considered to have zero value and is not included in city planning. In order to resolve this situation the estimation of the underground space is necessitated.

The present paper attempts to contribute to this issue by proposing an integrated methodology that estimates not only the market price but also the environmental benefits in monetary terms. The proposed methodology combines elements from the real-estate appraisal methodology and environmental economics in order to provide an estimate of the value of the underground space. The methodology was implemented to estimate the value of the underground space in the case of underground storage facilities in Athens, Greece. The results indicate that underground space has significant value and one third of its value is attributed to the monetized environmental benefits of the underground space. In general, the methodology is versatile and can be adapted to meet various needs and land-uses. Furthermore, it can be an effective tool in the hands of city planners, city authorities and other stakeholders leading to integrated city planning without neglecting the underground space and improved long-term management of the underground space.

城市化是世界范围内的既定趋势。它在城市地区产生负面影响，威胁其可持续性和未来。在这种情况下，城市正在加速利用地下空间。但是，通常只能将地下替代方案作为最后的选择。同时，地下空间被认为是零值，不包含在城市规划中。为了解决这种情况，需要估计地下空间。

本文试图通过提出一种综合方法来对此问题做出贡献，该方法不仅可以估算市场价格，而且可以估算货币方面的环境收益。所提出的方法结合了房地产评估方法和环境经济学中的要素，以提供对地下空间价值的估计。在希腊雅典，采用该方法估算地下空间的价值。结果表明，地下空间具有重大价值，其价值的三分之一归因于地下空间的货币化环境收益。通常，该方法是通用的，可以进行调整以满足各种需求和土地用途。此外，它可以成为城市规划者的有效工具，