The choices and the pros and cons before agreeing to purchase a made to measure door, we genuinely want you to purchase what best suits your home and your pocket, doors can be an expensive item when made to order, take your time in reading this as it may save you some hard earned cash.
1, Altering the opening to suit a standard size door
Width; The option to alter the door frame or opening to suit a standard size door really depends on how much you need to alter the opening, generally speaking (and in my opinion) you should only really reduce the opening in width by a maximum of 50mm each side when trying to use a standard size of door, any more than this will begin to look bulky and not very professional.
Height; Altering the opening height is much the same but if the standard door is eg; 150mm shorter than the door opening I would suggest you take a little bit more of the door height and then fir a "transom" rail above the door, a transom allows you to the then fit a small amount of glass above the transom, if the doorway already has a transom you could also have the choice of lowering the transom to suit the new door height.
2, Alter the door, making it larger to suit the opening
Width; My own rule of thumb is to add a maximum of 30mm or 15mm each side, any more will again look very bulky, always try and use the same sort of material as the door is made from, oak for oak, mahogany for mahogany etc etc.
Height; Adding to the height is also very limited but not impossible, add a maximum of 20mm to the top of the door and 40mm to the bottom, some people (joiners / carpenters) will now be throwing their hands up in the air with alarm at this suggestion but it really can be done and if the door is professionally decorated these additions will never be noticed.
3, Alter the door, making it smaller.
Width; Most standard size doors are veneered on solid timber sections and as such they need great care (not least with correct final decoration) when trying to cut in width, the easiest way is getting a professional joinery shop to do it for you as with the correct type of machinery the door can easily be reduced to allow about 20mm to be removed from each side. To do this properly you really need to remove about 32mm, then reduce this piece by the 20mm required before adding the trimmed down section back on to the door, glued and pinned then filled as required.
Height; A lot easier than the width, cut a maximum of 20mm from the top and the bottom, any more than this may impact on the type of construction joints that were used in the original manufacture of the door. The pros for choosing one of the above routes is that it generally works out cheaper, the downside is that it is a compromise and one needs to consider whether adjusting the opening to suit a standard size door or adjusting the door to suit the opening is doing what is right for your home, just remember you may need to live with it for some time, possibly a long time.
4, Making a door to size.
The main reason you would make a door to size is virtually always either aesthetics, property grade listings or due to restrictions with planning for whatever reason, sometimes planners don't appear to need a reason but that's another story.
The manufacturing of a one of a kind door is never cheap and neither should it be, it takes a great deal of skill to produce wooden doors "just for you" and the time for setting tooling to do so was or should I say is never cheap, the machine men are also quite rightly paid well for their skills.
The pros for choosing to have a door made is that you get what you asked for both in size and style, the downside is it will be more expensive.
DirectDoors.com at External Bespoke Doors with Glass Fitted or External Bespoke Panel Doors, either choice as a starter can make the doors shown there to size or provide traditional looking doors by way of a quote that won't incur the planners wrath.