New patio door dimensions do not match

[sha256]

I would like to purchase a new patio door; I measured the length and width of the door from the inside and it reads 2100mm (L) X 810mm (W).

Now when I look at doors at wickes/b&q they tend to read higher than the measurements for my door e.g. The width of the doors is 840mm, that is 30mm more than my door. Will I need to take out a portion of the wall to accommodate this dimensions ? or am I measuring my door incorrectly ? i.e. should the measurement be done on the outside?

 

[Nooz]

Your measurement will be from the outside normally. Take it from the edge of your existing frame to get it spot on. The ones sold in Wickes etc will be a standard size so don't be surprised if they don't match.

 

[Gazman16]

Get some prices from local double glazing company's. They will get them custom made to your sizes but will be much better quality than the stuff at Wicks. Price shouldn't be too much more for supply only.