I really feel sorry for you and I hope there will be an end of this problem.
I guess that your restoration of the house is making him confused and takes away the old scents and he has to make new ones.
We have several good Swedish cat sites and I’ve been reading suggestions for cat owners having the same problem as you have. One woman got rid of the problem by spraying perfume on the places were the cat sprayed. Another suggestion is to use ground white pepper. If you catch him just when he is spraying you can take him up and straight to the sandbox to show him were the right plays is. Don’t do this if the accident already is done. It will confuse him and he won’t understand what you’re doing it.
Feliway is sold here too, but I also have another spray (Swedish brand) that I used when Costello started to scratch on our couch. Started to use it a bit too late, though, when the couch was already ruined, but he sure hated that smell. We got another couch now in leather and he doesn’t touch it and I would be happy to send you the spray if you would like to try. Just PM or email me your address.
((((((((((HUGS))))))))))))
//Alice