In the recast Energy Efficiency Directive (EU)2023/1791, Article 3 makes Energy Efficiency First (EE1st) a fundamental principle of EU energy policy and requires Member States and competent authorities to: systematically assess energy efficiency solutions (including demand-side flexibility) in planning, policy and major investment decisions, and apply cost–benefit methodologies that capture wider benefits and address energy poverty. ENEFIRST Plus has developed a CBA Excel Tool focused on local heating planning in the residential sector, to illustrate how EE1st can be implemented in Cost-Benefit Analysis (CBA). The tool allows users to: Define a local building stock based on standard archetypes (building types and age classes). Compare a NoEff option set (continued use of gas, oil and biomass boilers with only standard envelope maintenance) with an EE1st option set (light and deep renovation of the building envelope combined with air-to-water and ground-source heat pumps). Calculate costs and benefits under both a societal perspective and a private perspective. Quantify the energy-efficiency gap by showing where the privately least-cost option diverges from the socially optimal one. Explore simple policy levers (grants, carbon prices, tariff changes, soft loans) that could close this gap and see how this affects public budgets. Everything is implemented in Excel, with pre-filled data for all EU-27 Member States (building archetypes, technology cost ranges, energy prices and emission factors) and a clear four-step workflow. Basic Excel skills are sufficient; no modelling background is required. A separate Technical Model Description document, available with the tool below, documents the model logic, data sources and limitations for users who need more detail. This tool complements a guidance on integrating EE1st in CBA.