I applied for Schengen visa thru Germany embassy for travel from 10th August to 24th August, staying maximum days in Germany (5). I got my visa for duration of stay as 15 days (same as itinerary). Some plans have changed at my end and I may plan to Visit Spain as well as part of the trip with Spain being the maximum stay duration. Can i apply for another Schengen visa at Spain embassy giving the exact reasons so that my effective duration of stay covers the next 7 days? OR do I have to request the German embassy for duration of stay extension?
Once (correctly) issued, a Schengen short-stay visa cannot be extended except in situations of force majeure (natural disaster, sudden illness, strikes etc) that physically prevent you from leaving the Schengen Area within the limits.
The way to do what you want to is to apply for another visa that will cover your additional visit in Spain. Since Spain is the main destination for the extra validity period you should file that with the Spanish consulate in your country of residence.