Strive for your addition to seamlessly flow & integrate harmoniously with your current home or cottage; both visually and functionally.
Your addition really should be indistinguishable from the original.
In some situations, an addition can even greatly improve the appearance of the ENTIRE cottage or home.
To design a seamless addition, you have to start with the exterior. It is very important that your rooflines work visually, and that the overall combined appearance of new and original is pleasing.
New rooflines locations relative to existing rooflines will have a HUGE influence on whether or not your addition looks like an add-on.
For a small addition (substantially smaller than the original structure) initially look at all the possible physical locations on the exterior where you could imagine a new roof fitting in with the existing roof AND look 'natural'.
It is critical to find locations where new roofs will visually look as though they belong; the goal is not just to find locations where the roofs work technically (ie water will not be trapped and be able to drain).
For quite large additions (ie doubling the size), the rooflines are still critical, but the initial starting point is to be look at the overall 3-D form (what architects call 'massing') - see below.
The overall 3-D form ('massing') is related to the rooflines, but is a little different. In looking at an addition, one needs to be conscious of the resulting overall 'massing'; making sure it will look harmonious.
What is appropriate will be dependant on the specific design of your existing structure; however in a lot of cases, the overall massing will look best if the tallest parts are in the central part of your home or cottage.
With a smaller addition, ensure that the 'massing' of the new does not overpower the original. Substantial additions (ie doubling the size) are much more involved; so unfortunately it is difficult to easily explain.
You've probably heard the term 'the devil is in the details'; especially true here. Doing the following will go a long way in visually helping your addition not look like an add-on.
Windows - in most cases, you should use the same style (casement, double-hung, ...), and sizes that are of a similar proportion to the existing windows.
Roof Slopes - in most cases (but not all), your new roofs should match the existing - or at least not look noticeably different. (But not a new Porch roof; it needs to be shallower, to look right.) Avoid flat roofs at all cost; a real easy fix - but they will look entirely out of place. If you have steep roofs now, but you think adding a shallow pitch is the only way; DON'T do it.
Roof Overhang - in most cases, match the existing dimension (not for a new porch)
Fascia Height (the part the eavestroughs attach to) - match the existing
Soffit Material (the horizontal underside surface of the overhang) - match the existing
Aside Note - If you have ice-damming on your existing roof, doing the above (same roof slope & overhang) will in-fact most likely replicate the same problem on your new roof - which you do not want to do. Further investigate your possibilities.
Once you have determined possible locations where the new rooflines will look right, you then can see how you may be able to get your functional requirements to work in any of those areas.
Think very generally at first, thru the filter of what would be ideal; how can you get the best use of space - as opposed to, 'what would fit here'.
Keep the big picture in mind - what would work best, what you ideally want, what would provide the most satisfaction.
In looking at your possibilities, always consider the impact on existing spaces, and how everything will flow together and relate. For example, one option may block a nice view, or another may mean an awkward relationship between 2 spaces.
Also realize that everything does not have to remain as it currently is, and some functions could possibly be shuffled around, which can open up other options.
It is very important that you are fully utilizing ALL of your space; not a lot of point building a big addition, only to infrequently use the space that your addition is to replace.
You really need to come up with a layout that makes use (ideally the BEST use) of ALL of your space. Doing so may even allow for a smaller and less expensive addition.