Domestic extensions - even entire new houses - are the way in which architects have traditionally started their careers. And the successful and established then often turn their back on this work, passing it on to colleagues who have just set up for themselves. The reason is that domestic work is so difficult and, properly budgeted, does not earn money. If you are a sole practitioner, with few overheads and plenty of time (ie not much work) then you can certainly earn something from it. But for a larger practice, with running costs that are substantial, it is likely to be at best a loss leader. The degree of complexity may be as great as in a project of five times the value, the client cares passionately and so is likely to change their mind, and their are likely to be unexpected discoveries plus all the difficulties of working with the small end of the building trade.
On the other hand, architect-designed extensions can be great, transforming not just the new space but the whole feel of a house. The best deal with levels and the all-important question of light. I saw a really good one yesterday, albeit on a house of a scale to which most could not aspire, which replaced a terrible off the shelf conservatory. Those conservatories are already a blight, and for larger extensions one could envisage other ill-conceived off the shelf solutions - perhaps going the other way, and being under- rather than over-glazed. Rooms which are currently well-lit could become poky and dark.
There is also the issue of paving over gardens, which may have serious implications for drainage on the larger scale if there are too many of these extensions.
Quick fixes are rarely simple.
