Balconies are a key feature of architectural design. Replacement of missing balconies, of a suitable design for the building architecture, is very likely to add to the desirability of the property and hence, the buildings overall value.
A visual inspection of the façade is likely to show markings or indents that indicate that once there was a balcony in place.
Measuring the distance between these marks should indicate the size of the missing balcony, and, it is quite likely there may be an existing one in the same geographical area that can be used as an example for a suitable pattern.
When restoring a missing balcony, a suitable pattern may be found in the same street.
Using the correct installation process will avoid most problems. Avoid using metals that are likely to corrode when in contact with each other. Don't use an inappropriate design, as that will detract from the building and reduce it's overall value.
Most balconies were manufactured from designs in standard pattern books (e.g. this book available from Amazon) available at the time of construction of the building. If you know the date the building was built, it is very likely you can find an appropriate design for the dimensions you have from these Pattern Books.