Which is better, more natural? "since the last year" or "since last year" Since last year, we have added more features. Since the last year that the scheme ran, we closed all the offices.

Since last year. It's not uncommon though to hear people using 'since the last year' in their conversations.

They are both correct, but they mean different things. "Last year" = "el año pasado", but "the last year" = "el último año". I haven't been to Montreal since last year = I went to Montreal during the year before this year. I haven't been to Montreal since the last year that I lived in Canada = I used to live in Canada, and then I moved away from Canada. The last time that I visited Montreal was during the last year that I lived in Canada. Maybe that was two years ago, maybe that was 10 years ago, maybe that was 50 years ago. If I haven't told you before in which year I moved away from Canada, then you don't know exactly how many years it has been.

I would suggest using "since last year". Honestly, "since the last year" (with the article) sounds ESL. I could see saying "in the last year", but not "since the last year".

you can say it both ways , but " since last year" sounds more natural.