Class C is the yardstick: reading the A to G label on a Czech building

The label looks simple: a ladder of seven rungs from green to red, with an arrow beside one of them. A buyer reads it in a second. What the label cannot say in a second is what the arrow is being compared with, and that is where most misunderstandings begin. An A is not a promise of no heating bill, and a G is not a verdict that a building is unlivable. Both are positions on a scale that is built in a particular way.

This explainer decodes the scale. For the certificate that carries it, see energy performance certificates explained.

The seven classes

The Czech scale has seven classes, each with a descriptive name in the decree.

The names tell you the intended reading. A, B and C are the good end. D is below the middle, and E to G are the poor end.

What the classes are measured against

Classes are not absolute. They compare the calculated energy need of a building with that of a reference building: a notional building of the same shape and use, built to a set of standard specifications for insulation, windows and technical systems. The reference corresponds to the middle of the scale, class C. A building that needs about as much energy as its reference is in C. One that needs much less moves up to B or A. One that needs more falls to D and below, in steps set by the decree (as of 2026; check the current decree for the thresholds).

This design has two consequences. First, a small flat and a large house are comparable on the scale, because each is compared with its own reference, so size alone does not decide the class. Second, the class says nothing about the absolute amount of energy used. Two buildings of equal class but different size use different amounts in total.

What moves a building up or down

The calculation looks at the envelope and the systems.

• Insulation: thicker or better insulation in walls, roof and floor reduces heat loss. In an older building, this is usually the biggest single lever.
• Windows and doors: modern glazing and airtight fitting cut losses; leaky or single-glazed windows raise them sharply.
• Compactness: a building with a small surface for its volume loses less heat than a sprawling one. A mid-terrace block does better than a detached house of the same floor area.
• Heating source and distribution: the type of heat generation, how it is controlled and how it is distributed matter. Condensing boilers, heat pumps and district heating each enter the calculation differently.
• Hot water: how it is produced and stored counts, since in a well-insulated building it can be a large share of the total.
• Ventilation: whether there is heat recovery matters in new, airtight buildings.
• Sun and shading: solar gains in winter and overheating risk in summer enter the calculation, particularly for modern glazed buildings.

The calculation also uses conversion factors that express how much primary energy is needed to deliver each unit of energy to the building, and how much of that comes from non-renewable sources. A building heated by electricity from renewable sources or by a heat pump can score well on non-renewable energy even if its delivered energy is not tiny. District heating is rated according to the properties of the particular network, which is why two buildings with the same envelope can differ in class depending on the heat source; Czech heating systems explained describes the options.

New buildings and old ones

Czech rules require new buildings to meet a nearly zero-energy standard, which was phased in from around 2020, with public buildings earlier (check the Ministry of Industry and Trade for the current requirement). In practice, that places new buildings at the upper end of the scale. A listing for a flat in a new development will therefore normally show a high class, and the interest lies in the gap between the top classes and in how the figure was calculated.

Older buildings span the whole range. A panel block that has been insulated, with new windows and a renewed heating system, can reach a high class. An unrenovated one will sit lower. Pre-war brick buildings often have thick walls with decent thermal mass but poor windows and roofs, and their class depends on what has been done to them. The condition of a building matters more than its age.

A building is not required to be upgraded merely because its class is low. The class becomes relevant when a building is sold or let, when major work is carried out or when policy tightens. For owners in blocks, an upgrade is a collective decision, and the large works described in panel block renovation are decided by the association.

Reading the label sensibly

The letter is a start. Beyond it, the certificate gives the calculated energy per square metre, the breakdown by use and the recommendations. For a decision, three comparisons help more than the letter alone.

Common misreadings

Three misunderstandings recur. The first is that class A means a building with no heating costs. It means a low calculated need, and a large house in class A can still cost more to heat than a small flat in class C. The second is that the class describes comfort. A building can score well on energy and still overheat in summer, or have poor air quality if ventilation is badly designed. The third is that a low class is always a bargain waiting to be improved. Improvement costs money, takes the agreement of co-owners in a block and may be restricted in a heritage setting, as heritage protection and building restrictions describes.

What is changing

The EU's recast directive on the energy performance of buildings, adopted in 2024 as Directive (EU) 2024/1275, changes the logic of certificates across the Union. It moves towards a common A to G scale in which the top class corresponds to zero-emission buildings and the bottom class to the worst-performing part of the national stock, and it sets targets for reducing the average energy use of the residential stock over the coming decade (as of 2026; check EUR-Lex and the Ministry of Industry and Trade). Member states must transpose it into national law, and how Czechia does so will determine whether and when the Czech classes are redefined. Until then, the current decree governs, and a certificate issued under it remains valid for its term.

For owners, the practical message is to treat the class as a living indicator. A building that scores in the middle today may look weaker on a recalibrated scale, and one that has already been improved holds its value better.

Frequently asked questions

What does class C mean?

It corresponds to a building that needs about as much energy as the reference building. It sits in the middle of the scale.

Is a G-rated building illegal to live in?

No. The class is an indicator of calculated energy need. It matters at sale or letting, for major works and for policy, but it does not by itself bar occupation.

Why do two similar flats have different classes?

Differences in insulation, windows, heating source, hot water, shape and orientation all affect the calculation, as does how complete the data was.

Will the scale change?

The EU's 2024 directive points to a redefined scale, and national implementation will decide when and how. Check the Ministry of Industry and Trade for the current position.

When you compare two listings, put the class beside the heating source and the last year's real bills. The letter alone will not tell you how the building behaves in January.

This is a general explainer, not legal or tax advice.