How can you add 3 floors to an old house?

[Suhagg]

Now I'm not talking about villas but, for example, store/office buildings in Sthlm.

I've seen several 3-story buildings where they just add 3 more stories just like that. How does this hold, and how do you calculate this?

 

[JanneL]

Fully possible to do. However, not just like that. An engineer needs to calculate this.
I assume it needs to be reinforced from the basement all the way up.

 

[Mats]

One must calculate properly on such exercises and check that the ground can take the weight, and then, as mentioned, reinforce upwards floor by floor. Built an additional floor at work, and it was a really big operation with everything surrounding it.