Hi Ivan,
QUOTE:
As far as I can see I need to edit gui/extended/details.php file.
thatīs 100% exact.
You will have to add extra fields there (for example input type text, radio, checkbox and so on) and then edit the function sendMail() in estateagent.php between line 656 and 688.
Please take a look at this function. I think, it is selfexplaining how to extend it with more fields.