What Is the Building Importance Factor? What Does 1.5 Mean in Housing?

When people speak of an earthquake-resistant home, most first think of concrete quality or whether the building is new. Yet behind the scenes there is a more fundamental decision that directly determines seismic safety: the building importance factor.

This concept rarely appears in listings — often it is never mentioned at all. But for anyone who wants to understand how a structure will behave in an earthquake, it is one of the clearest indicators.

For a more detailed look at the criteria used to evaluate an earthquake-resistant home, see our article on what an earthquake-resistant home is and how real safety can be recognised.

What does the building importance factor tell us?

The building importance factor shows how far on the safe side a structure's seismic calculations were made. In other words, engineers use this factor to determine the multiplier applied to earthquake loads when designing the building.

Put briefly: the higher the building importance factor, the larger the earthquake the structure is calculated for. That means everything from the load-bearing system to the reinforced concrete elements is designed with higher safety margins.

Are there different levels of this factor?

Yes — the building importance factor is not a single number. The Turkish Building Earthquake Regulation defines different levels according to a building's intended use.

The great majority of standard housing is designed with a building importance factor of 1.0. This value meets the minimum safety conditions foreseen by the regulation and is legally sufficient.

For buildings considered more critical for society, the factor rises to between 1.2 and 1.4. Educational buildings, crowded-use areas and some public buildings fall into this group.

At the top level sits the 1.5 building importance factor. This value is used for structures that must remain standing and functional even after an earthquake, such as hospitals and emergency facilities. Here the goal is not merely to avoid collapse, but to remain usable after the earthquake.

Why is 1.5 a different choice in housing?

Choosing a building importance factor of 1.5 in residential projects is not an obligation. On the contrary, it is an entirely conscious choice, seen in projects that aim to exceed the standards and treat safety at the highest level.

In homes designed with a 1.5 building importance factor, earthquake loads are taken roughly fifty percent higher. This means a stronger load-bearing system, increased safety factors, and potential damage limits kept far lower.

Where do the ground and the architecture fit in?

However strong the calculations on paper, the picture is incomplete if ground quality is ignored. Solid, rocky ground is among the most advantageous ground types for seismic behaviour. Transmitting earthquake waves without amplification helps the structure behave more predictably.

Architectural choice is also an important part of this equation. Buildings designed with horizontal architecture distribute earthquake loads more evenly. Compared with high-rise structures, their mass behaviour is more controlled, which positively affects seismic performance.

How was this approach handled at Royal Silva?

At Royal Silva, the building importance factor was treated not as a marketing line but as the project's fundamental safety decision. The buildings were designed on the basis of a 1.5 building importance factor, with an engineering approach at hospital standards.

Set on rocky ground, the project was considered together with its horizontal architecture. The aim was to create a structural system that behaves in a controlled way during an earthquake and allows life to continue safely afterwards. At Royal Silva, safety was not an added feature — it was the project's starting point.

So what should we look for?

An earthquake-resistant home is not simply a building said to be 'uncollapsible'. What matters is that people feel safe during and after an earthquake. The building importance factor is one of the clearest technical indicators of this approach.

Questioning such details when buying a home means securing not only today but the future. Because the real investment is the one made in safety before square metres.