It is the law in all Schengen countries, but the criteria for what is considered serious personal reason seems to be vague, and up to the member state to determine. From their website, it appears as though Sweden is more lenient than some others... Poland also appears to be more relaxed. Some countries seem to be really tough... In the CZ, local visa agents are saying that they pretty much only do this extension if you are on fire.
For Poland, it is stated: "When it comes to the optional visa extension – you have to present the authorities with compelling personal reasons that justify the extension of your stay in Poland. The fee for this procedure is EUR 30 (or its equivalent in Polish złoty)."
That's it... No qualifiers for what is compelling or justifiable.
In this case, I am attending an event that got moved a week in to the future, and she is traveling with me. Is that serious? Who makes the determination?
Maybe not in France, but many countries in the EU have information about an extension... Most say it must be "exceptional" circumstances, but others do not... It is quite explicit that there is something in the Schengen Visa Code, but I can not find this in English...
CZ Immigration Site:
"Conversely, applications for extension of short-term visas for the territory of the Czech Republic(only in exceptional circumstances that are stipulated in advance by the Visa Code) must be made at the department of stay and residence agenda at a department of Alien Police of a regional Directorate of the Police of the Czech Republic according to the third country national’s registered address."
Holland:
Conditions for extending a short-stay Schengen visa
To be allowed to extend your visa, you must meet the following conditions:
Your stay in the Schengen Area must not exceed 90 days in total. If you stay longer than 90 days, your visa will be limited to the Netherlands.
You must explain why you need to extend your stay. If you are unable to return to your own country before your visa expires, you will need to show documents to demonstrate why.
You must have enough money for your stay in the Netherlands (at least €34 a day). Or you can ask someone to act as a guarantor.
You must have valid travel and/or health insurance.
Your passport must be valid for at least another 6 months and must not be more than 10 years old.
There must be no indications that you have other reasons for extending your visa, for example illegal residence in the Netherlands.