Stephane,
First, SAP is executing the availability check at plant level and no quantity is available at plant level, SAP doesn't go on at Storage location level.
Not exactly. SAP standard ATP check generally checks all valid segments: 'Plant', 'Storage location' (if specified), and 'batch' (if specified). The results of the ATP check are the LOWEST of the three checks. You are definitely right that it is virtually impossible to get SAP standard ATP check to ignore 'plant level' stock quantities in preference to 'storage location level' stock quantities under standard plant level planning scenarios.
A small exception exists; you may want to experiment with planning the storage locations separately. MM02 > MRP4 tab > Sloc MRP indicator = '2'. In this case, each storage location acts more or less like a plant under ATP. There are other caveats under this scenario, proceed cautiously.
If planning storage locations separately does not meet your requirements, then you would need to implement an ATP enhancement. Proceed cautiously. ATP enhancements tend to have the nasty habit of generating overconfirmations, unless they are carefully tested under all possible scenarios.
Best Regards,
DB49