Planning permission was given for the expansion of the ground in Fenruary 2014. Preparatory works started in March 2014, with more substantive works at the end of the season. It was completed in August 2015. IIRC, they had to really rush in the last few days to be ready for the first home game against Chelsea.
In theory, they do have planning permission for the North Stand already but if there are changes to the original plans I suppose they might have to resubmit the planning application. I'm wondering if this is what worsley can't tell us, though I do appreciate that you won't be able to disclose even this.
Anyway, a start in New Year 2017 would seem to presuppose completion in summer 2018. IIRC, during construction of the South Stand, we hosted concerts by One Direction in the summer of 2014 when the work was in its early stages but none in 2015 as the works neared completion.
If they do the North according to the above timetable and follow the same pattern with the concerts, we may have some in 2017 but there'll be none in 2018.