Introduction

The Renovation Twin of the city of Plovdiv is a novel, 3D Geographic Information System (GIS)-based digital platform that brings the city’s energy future to life. It is also a forward-looking instrument aligned with the latest requirements of the European Performance of Buildings Directive (EPBD) and its emphasis on digitalisation, data transparency, and strategic renovation planning. The Twin proactively fulfils key EPBD requirements by establishing a functional, scalable framework for digital building logbooks (DBLs), serving as a dynamic building stock observatory (BSO), and creating a comprehensive, predictive Energy Performance Certificate (EPC) database.

The platform transforms fragmented public data into well-structured DBLs, a key requirement under Article 19 of the EPBD. These logbooks aggregate critical technical, energy, and renovation data into a secure, accessible digital record for each building, providing the foundation for lifelong building documentation. In addition, the Twin itself embodies the EPBD’s call for national building observatories. It offers local authorities a powerful, data-driven dashboard to monitor stock performance, prioritise renovation efforts, and strategically plan urban regeneration, supporting evidence-based policymaking and reporting.

Crucially, the platform directly addresses the EPBD's drive for reliable and accessible EPC data. Its intelligent ‘living’ algorithm not only integrates official EPC records but also generates reliable predictive energy classes for buildings lacking certificates. This creates a complete, city-wide EPC database, filling critical information gaps and ensuring a comprehensive overview essential for setting robust renovation targets and tracking progress.

Looking ahead, the Twin’s scalable architecture is designed to seamlessly integrate future EPBD provisions, such as building renovation passports and Smart Readiness Indicators. By building a verifiable digital foundation today, the Renovation Twin positions the city of Plovdiv not only to comply with but to leverage EU directives, turning regulatory requirements into a practical engine for accelerated, data-driven urban energy transition.

The challenge: market stagnation against urgent renovation needs 

Since 2012, Bulgaria’s National Renovation Programme has renovated less than 2% of all dwellings in the Plovdiv Region while national statistics suggest that nearly two-thirds remain in urgent need of renovation. This slow progress occurs even as estimates reveal a massive opportunity: they suggest that with more ambitious, systemic action, the Plovdiv Region could achieve over 65% energy savings and a 35% reduction in CO2 emissions.

_{Renovated buildings in Plovdiv City, Bulgaria (source: Energy Agency of Plovdiv)}

The overly generous 100% state grant has created a public fixation on public funding, stifling private investment and mainstreaming moderate renovation ambitions. This has dulled awareness of the full benefits of renovation and discouraged homeowners from pursuing innovative, deep energy solutions.

The solution: a data-driven digital city twin to unlock renovation potential 

The Energy Agency of Plovdiv (EAP) developed the 3D Renovation Twin of the city of Plovdiv within the co-funded LIFE programme MultiHome project to directly address this stagnation, aiming to accelerate renovation rates by providing homeowners and municipal officials with an innovative, data-driven, and user-friendly decision support platform for their renovation plans. 

The Renovation Twin is an innovative GIS-based 3D digital twin of Plovdiv’s residential building stock, providing homeowners with one-click information on their building’s energy performance. The platform also features powerful interactive tools for homeowners, the Renovation Motivator and Solar Simulator, to support informed decision-making throughout the renovation process. On the other hand, it is a powerful renovation planning tool for municipal experts that highlights the demand for renovations across the city and outlines potential renovation pipelines.

Beyond its appealing visual looks, the Renovation Twin is a powerful BSO that aggregates and synchronises disparate data flows into a single platform, with a digital logbook for every building that integrates its technical and energy data. At its heart, the Twin has a ‘beating’ EPC algorithm engineered by the Energy Agency of Plovdiv that predicts energy performance and powers interactive tools like Renovation Motivator and Solar Simulator. Designed to be GDPR-compliant and scalable, the Twin’s flexible architecture makes it a replicable model for other Bulgarian and EU cities.

The Renovation Twin: a breakthrough innovation enabling residential renovation

The Renovation Twin represents a landmark in data-driven policymaking for Plovdiv as it integrates pioneering solutions to some of the most persistent challenges in building stock management: fragmented data, inconsistent standards, and static analytical models. Its innovation lies not in any single component but in the holistic and intelligent integration of disaggregated datasets into a dynamic, scalable, and user-centric platform. 

The EAP addressed a series of cascading challenges, leading to the final 3D platform of the Renovation Twin.

Overcoming data fragmentation through intelligent aggregation

The initial challenge was a classic ‘data scatter’ – building information existed across multiple institutions and formats and did not have a usable unified form. The team established ‘data anchors’ enabling the cross-referencing and merging of records to create a singular, comprehensive BSO. Furthermore, EAP sourced and heavily aggregated data from various institutions to conceptualise and assemble individual DBL for each building. This process of creating a unified data backbone from mismatched sources was fundamental in transforming scattered public data into a structured asset.

Powering the Twin: a dynamic and scalable EPC algorithm

The beating heart of the Twin is a groundbreaking living EPC algorithm. Unlike conventional tools, it does not produce a one-time assessment. It dynamically adjusts its forecasts as new data is entered into the logbooks, ensuring ever-greater accuracy. When new data is entered into the system, whether from a new physical EPC, updated database figures, or user corrections, the algorithm refines its predictions. This creates a virtuous cycle; the tool grows more accurate and trustworthy as it is used, ensuring its long-term relevance and establishing a continuously improving knowledge base for the city’s building stock. Furthermore, it is generalised enough to be applied to the entire building stock yet differentiated enough to account for specific building features with high accuracy.

_{Renovated buildings in Plovdiv City, Bulgaria (source: Energy Agency of Plovdiv)}

With its powerful EPC algorithm, the Twin fills in data gaps for EPCs for thousands of residential buildings in Plovdiv.  This predictive capability transforms the platform from a repository of known information into an active intelligence tool, providing a complete picture of the city’s energy landscape where none previously existed. It empowers city planners and homeowners to make decisions based on a comprehensive dataset, not just a small sample of audited buildings.

User-centric modules for tangible impact

The Renovation Twin harnesses the power of the EPC algorithm in two public self-service tools: the Renovation Motivator and the Solar Simulator. They are innovative modules that translate raw algorithm outputs into personalised, actionable insights. They take predicted EPC data and dynamically simulate renovation effects, cost implications, and solar energy potential. This demystifies complex technical and financial calculations, placing the power of advanced building physics directly into the hands of homeowners. The shift from an opaque, hour-long consultancy to a transparent, one-minute self-assessment is a radical innovation in citizen engagement and service delivery.

Validation and integrity of the Renovation Twin

The EAP has validated the Renovation Twin in several ways to prove its value and performance, through national statistical projections, benchmarking against existing EPCs, assessing outliers and comparing results with other simulation tools. The validation results underscore the tool’s effectiveness and pragmatic accuracy. While achieving exact class alignment (~30% of cases) is challenging due to variances in real EPC methodologies, the algorithm demonstrates over 90% accuracy within a two-class range, more than sufficient for prioritisation, awareness-raising, and initial planning. Notably, the system’s highest accuracy aligns with the most common building classes (E and F), ensuring reliability where it matters most for mass renovation. This transparent validation process is an innovation in governance, ensuring the tool remains credible, accountable, and scientifically sound.

Versatile applications for homeowners and city planners 

The Renovation Twin is a uniquely versatile and scalable platform, designed to serve as a critical decision-support tool for both individual homeowners and city planners, bridging the gap between personal decision-making and large-scale urban renovation planning. For homeowners, it demystifies the renovation journey through intuitive self-service tools like the Renovation Motivator, allowing them to visually assess their building's energy performance and simulate the impacts and savings of potential upgrades in minutes. For municipal authorities, energy agencies, and national bodies, the Twin transforms into a powerful data-driven management system. It enables municipal officials to identify, prioritise, and build pipelines of the neediest buildings for targeted campaigns or funding programmes, monitor city-wide renovation progress against targets, and strategically plan large-scale regeneration projects, such as developing a Renovation Roadmap for the entire Plovdiv Region. By seamlessly integrating a public-facing empowerment tool with a robust analytical backend, the Renovation Twin functions as both a catalyst for individual action and an essential instrument for systemic change, turning fragmented data into coordinated renovation progress. 

Future expansion of the Renovation Twin

The Renovation Twin was not developed as a static tool, but as a dynamic and expandable digital ecosystem, with its powerful foundation already paving the way for significant future development. Its core strength lies in the integrated ‘living’ data of the DBLs and simulation engines, which have the potential for versatile and seamless expansion. The immediate next step is the deployment of a BSO dashboard for municipal officials and energy planners. It will provide direct access to detailed building logbooks, encompassing a technical passport, energy profile, renovation status, smart readiness, and lifecycle assessment, all in one place. This expansion will empower data-driven governance and enable precise monitoring, reporting, and strategic planning of major renovation projects. Furthermore, the Twin’s modules will be further developed to integrate financial instruments, grid-related functionalities, and renovation pipelines. This will ensure that the Renovation Twin will evolve to a central decision-support tool for Plovdiv’s long-term sustainable urban transformation.

Conclusions

The Renovation Twin of the city of Plovdiv is a groundbreaking development engineered by the EAP in support of long-term sustainable renovation planning for homeowners and municipal officials. Its development is not only a technical feat in data fusion and dynamic algorithm design but also a breakthrough in creating a public asset from disjointed data that may revolutionise the way the Renovation Wave is implemented within the community. 

By creating a system that learns, adapts, and empowers, the EAP has not just built a tool for Plovdiv but has provided a validated, scalable blueprint for any city aiming to harness its data to accelerate its sustainable energy transition. The Twin is a living testament to the power of integrated innovation, turning urban data into a catalyst for real-world action.