Sorry I can't offer much help but I had the same problem a few years ago, there's only so much you can plane off the door otherwise when summer comes around there'll be a great big gap around the door. I ended up biting the bullet and replacing it with a UPVC door.
Where is it sticking , Side or bottom ? Id be tempted to take the door off and plane 1mm off bottom and side with a power plane and rehang the door ,If you plane of the hinge side and chop the hinge in you wont have to mess about with the locks etc . If you don't have a power plane homebase do one for about 20quid that'll do this one off job .
Dave has the door been painted along its top and bottom edges? Most of the time I see this sort of problem the door is untreated and this allows rapid moisture change in the stiles. Not a good thing :roll:
Some years ago I had the same problem with our front door which faces the English Channel.
So I upset the wife and fitted a large sheet of poloythene over the door frame that winter and if anyone wanted to come in they had to come round to the back door.
But, I had good intentions, because I then fitted out a porch and was able to keep the front door.(and free of moisture)
So a porch might be your salvation (with a plastic door )
I have 4 interior doors that do the opposite, They are doors with 4 panels in them top two panels have etched glass in them, the bottom panels have raised panel inserts, ( I was going to make them myself until I seen these in Wickes) they were made in South Africa the price of about £24.00 per door included the glass fitted about 4 years ago, In the summer they swell and in the winter they revert to normal, I will not bother trimming them, We have learned to live with the problem, I have hung hundreds of doors and have never came across this phenomenom, anyone any ideas what causes it
A memory just came to me, about 20 years ago I built a porch for a customer, When it came to the door I gave the chap a catalogue from a firm that makes hardwood doors with Mortice and Tenon construction, the price for the door he wanted was in the region of £180.00 then, he said he would not pay that much and said that he would find a cheaper one, well he got one and when I seen it I told him it was not good enough for an outside door, he said "just hang it it will do me" I gave it two coats of outdoor yacht varnish, let it dry and hung it. I went on holiday a few days later for 2 weeks, a week or so after I got back he called me and asked me to come and fix the door as it was sticking, I took one look at and told that is what happens to cheap doors, he argued and to shut him up , I put a pair of sash cramps on the door top & bottom tightened them up a midges, released the cramps and the door closed as sweet as anything.
The moral is for a good outside door that will give no problems for 20 or more years have one made with M&T joinery,
The ones you buy at the big box stores are invariably made with dowel construction and they tend to come apart in any type of weather.
Maybe this is what you have. Check if have any gaps between the stiles and the rails, even minute gaps will cause the door to stick, also has the door had the leading edge champered.
Dave, I agree with keith, have you sealed the door on all 4 edges and also the letterbox & inside the lock & handles, especially if the door is a far eastern hardwood door, these are generally made from one of the softer grainer hardwoods and are prone to absorbing more than their fair share of moisture. Having fitted lots of these doors in the past, I found that making sure all the bare wood is treated with a suitable sealer.