While the Japanese Maple might start small and skinny, they are not going to remain so forever. I would think of them as being at least a 5' ball and later, a 5' ball on a stalk. Later than that, they will be larger still. I can't see anyplace that such a form would fit in this bed. In fact, I'd be getting rid of that fat shrub at the left sticking way up above the window. (That's not an enhancement to the window.) Then, that lopped off tree at the corner should be allowed to grow into a tree ... much higher with the lower branches removed. in the end, you should be able to see below its canopy. The roses would be OK in that position, but not with those trellises. The trellises should be beefier and reach the same height as the top of the windows. The shrubs that you plant below the windows should get no more than 30" height. Everything else in the bed should be low ... 12" ht. or less.

I disagree - dwarf Japanese maples (as all these are) are often contained in beds far smaller than this. And there is no rule that states that all plants have to remain below the window height - obviously you don't want to obscure them completely but a portion of a tree, especially a very lacy, deciduous one, covering up part of a window is NBD.

I do think all three in this area is too much. 'Baldsmith' would be my first choice as it is short weeper and very appropriate for a front garden plantings. 'Kamagata' is also quite dwarf and runs a close second. 'Shaina' I would locate elsewhere.

I'm not sure what direction the entry faces but it doesn't look like its enough sun for the roses. And if that's the holly right next to the porch, I'd get rid of that as well. Lower growing shrubs could fill in the area nicely, with a mix of some smaller grasses and/or perennials. Again, I wouldn't necessarily place an arbitrary limit on the height - there is really no basis for such a restriction. FWIW, other than a few annuals or groundcovers, there aren't many plants that stay below 12"

"...there is no rule that states that all plants have to remain below the window height..." ..."I wouldn't necessarily place an arbitrary limit on the height - there is really no basis for such a restriction. FWIW, other than a few annuals or groundcovers, there aren't many plants that stay below 12"

Actually, the restriction against covering windows (or other architectural details) is not arbitrary; there is a consequence to doing or not doing. When designing against a the wall of a building, we are employing many of the same principles of graphic arts, primarily and most commonly, the relationship between 'objects' and 'white space.' There is not time here to explain how the relationship between white space and objects works, but it is a long accepted operational theory of art that anyone can easily research and decide if they accept, or not. To quote Wikipedia, "A page crammed full of text or graphics with very little white space runs the risk of appearing busy or cluttered, and is typically difficult to read." This sums up the consequence of not paying adequate attention to the value of white space and applies equally and in every way to landscape design (... or any design really.)

At the face of a house, the wall typically works as white space and all those architectural objects: windows, doors, columns, trim, corners, edges, shutters, etc -- and plants -- are objects that compete for their share of it. It is when some objects cover others that the danger their being "read" as visual confusion occurs. If the covering objects seem misshapen or unkempt, it is almost a certainty that the picture will have a negative effect on the viewer instead of a positive one. And it's a rarity when shrubs cover windows if this doesn't occur. (It's occurring at the left side of the above picture.) The more that the "good" object (anything that's supposed to be there ... like a window) is covered up, the more it looks like someone has made a mistake. The degree of the mistake correlates directly to how much the object is covered and how homely the object doing the covering is. For myself, I can hardly look at a window covered by overgrown shrubbery without thinking it's a home where live the heavily medicated elderly. It brings about the picture and smell of a vitamin B bottle spilled on the dresser.

An artist can purposely and successfully break this rule to achieve a specific conscious effect. However, most people asking for advice here aren't apt to know how to break this rule to that effect. And, of course, there is a difference between "touching" and "covering." There are many times plants cover a small portion of an object to great effect.

As you point out, gardengal, many shrubs do not stay low enough. At many homes, the architectural details reside easily below 3'. To my thinking this arrangement represents innumerable examples where lower plants -- perennials, groundcover and even some annuals -- are much better solutions if one doesn't want the obligation of frequent trimming.

I have posted another photo so you can see the front of the house. You all make good points. It never occurred to me about the shrubs going over the window. I think they would look better lower and I will trim them down as soon as they bloom. These are rhododendrons and are going to burst forth any day now.
As far as the roses, they could use more sun. They have been there about 6 years. They look beautiful when they bloom in the spring and then they look sickly for the rest of the season until winter. They are thorny and a pain to maintain. If I planted 1 or 2 dwarf maples, how would you lay them out in this bed?

Odd that with the schooling behind two different design degrees this specific height construct - which I believe to be very arbitrary - was never pointed out. Nor have I ever seen it outlined in any text that discussed design principles. I am very familiar with the concept of positive and negative spaces......in fact I lead a rather detailed discussion of it several years ago on this forum......but I am not sure I see much relevance to it in this instance.

As this portion of the garden is not the focal point, I see no reason why architectural details should rule the day. As long as the light/view from the inside out through the window is not obscured to the homeowner's taste, height has no bearing. How tall a plant gets is only a matter of choice and with a caveat of a scale that suits the situation. If in doubt, I believe it is far better to err on the size of a larger, more substantial plant than one that is puny or undersized. They don't show up, they don't make any sort of statement, so what's the point? You might as well run the lawn right up to the siding!

The difference between our thinking, GG, might be that you accept the status quo of design -- what has been taught, written about and said by "leaders" in the field as a body of knowledge that is relatively complete. I think it's in a floundering, infantile stage, and accept little of it as being complete in its development. I think there is much to be worked out and see a distinct difference between developing a "garden" and a "landscape," in which case planting that uses a wall face as an immediate background, architectural details can never be ignored ... despite what remains unsaid (by so called "leaders") about their importance or lack thereof. While a focal point is an area of increased importance, its existence does not mean to me that other areas become unimportant and therefore, coverable. As I mentioned, I believe the "white space" theorem applies equally to all design in which the artistic outcome matters ... even the gastronomical arts. If a designer determines that architecture or subordinate features aren't important enough that their being seen matters, then he has the prerogative, or possibly the obligation, if said features are sufficiently homely, to cover them. But for the average door, window, shutter, etc. I don't think that's the case. It seems a trend that many new homes are built showing features of interest nearly to grade. There are many times when the best choice is to run the lawn all the way to the wall. (This house is not one of those times, there being excessive "white space" below the windows.)

"I will trim them down as soon as they bloom. These are rhododendrons and are going to burst forth any day now." I would not only trim, but find a new home for a plant that wants to be much larger/taller than the space warrants. This is one of those things that makes a difference between maintenance being high or low when multiplied by several plants around the yard.

I cannot answer your question about how the maples should be arranged in the bed. Someone who agrees with their being there might.