Architectural Studio Apxe

Private investor
Studio APXE Ltd.
Georgi Nikolov (lead architect), Silvia Draganova, Boshko Takov
Project management: Studio APXE Ltd.; Constr companies: K V Engineering; Schloss; NG Technology, AByU Engineering, Bultherm, Muharski, DSV, Citybuild studio, Filkab and many more; Photo credits: Studio APXE and Missirkov/Bogdanov
Sq. Area:
970 m2
Home / Projects / Plus energy house in Delta Hill, Kladnica

Plus energy house in Delta Hill, Kladnica 

Тhe family passive house is situated in Delta Hill, the vicinity of Kladnica village, the south green slopes of Vitosha mountain in a picturesque area.

The headway and experience gained of the Studio APXE team contributed for our most significant success till now.

The challenge was to fit successfully the large scale one-storey building in the green landscape without appearing foreign and superfluous there. We detached by their volumes and structure the garage and the cold service premises from the residential part. This solved many technological and energy problems with the external surface thermal bridges. The volume of the garage is fitted at street level, dug into the rock and the house is partially over it, standing broad.

The building was built with a lightweight ecological load bearing structure. Only the foundations, the supporting frames and the roof of the garage are made of reinforced concrete. Everything else is designed with a lightweight steel and timber structures. This lightens the weight of the structure and reduces the budget as well. Laminated wood beams and columns pergola with built-in sun protection form the appearance and the elements of the architectural envelope.

The house is designed in accordance with the Passivhaus standard. Here are the energy parameters calculated in PHPP:

  • The specific heating energy demand is 12 kWh/m2 annually;

  • The need for primary energy for all purposes is 38 kWh/m2 annually. This is significantly lower than the maximum allowed value for a passive house 120 kWh/m2 annually. We achieved this value by replacing the heat source with a biomass one, which is yet another contribution to the green concept of the building because of reducing  the coefficient for primary energy;

  • Excellent air-tightness level with a result from Blower door test n50: 0.13 h-1 on the preliminary and 0.19 h-1 on the final test. (Hundreds of details were designed during construction in order to get a satisfactory result under the required 0.6 h-1 for a passive house);

  • The house needs no cooling in summer.

How did we achieve these results in the design? With excellent insulation, optimized compact size and careful balance of glazing, with PHPP calculations, sun protection, reduced or no thermal bridges, innovative electricity services and lighting, and last but not least - a ventilation system with heat recovery with the highest characteristics.


The house will generate more energy than will consume per year. This makes it not just passive but an energy-plus building. This will be achieved by the installed on the roof 12.25 kWp photovoltaic panels. They will charge the battery station of the backup system of the house, which allows the generated energy to be used in the everyday household and the surplus during the summer months to be delivered via the grid. There are also 12 units of solar collectors which will mainly heat the swimming pool in summer, while during wintertime will assist the heating system.

The estimated budget for the construction amounts to 953 €/m2, which is our next success having in mind the high quality of the construction materials and systems. During the construction we optimized and reduced the costs through better management of processes. The additional investments to reach the "passivhaus" standard are calculated to 5,93% from the overall construction costs and further to reach positive energy balance with the plus-energy standard - 9.24% from the overall construction costs.

What is the outcome? - A beautiful and comfortable house which provides joy and comfort to the residents, fresh air inside, smooth, very cheap and easy operation. А reliable and truly comfortable home complying with the most advanced international environmental standards.

Look through the the passive house also on Arhitizer.

Read an article on Designboom.

Read an article on Gizmag.

Read an article on Inhabitat.

The Plus Energy house in Kladnica was awarded with Isover Energy Efficiency The Best of Awards (

Find more for the pressurisation "Blowerdoor" test on 01.02.2013.