When carrying out an architecture project, it is preferable to make a global intervention from the beginning, taking into account all the factors. This will help the entire project to be consistent and work properly. If you are looking to obtain the Passivhaus certification, it is highly recommended to do it from the beginning since only then can you guarantee that you will obtain the certification.
On the other hand, there is also the option of acting in the middle of the construction process or even intervening in an existing construction such as rehabilitation, and only intervening in specific elements and not in the totality. In the latter case, the type of certification that could be obtained would be EnerPHit, which is a specific certification for renovations.
It is necessary to be very meticulous in those type of interventions to guarantee the success of the project there must be coherence between all the elements that compose it.
That is why it will always be very important that the technician who designs and supervises the correct execution is well trained and is a specialist in the field.