OK I admit, I had to web-search the opposite of 'improvements'. My Chambers dictionary suggests ''to decrease the value of, by a given amount' .

You may well ask where I'm going with this. I sometimes do wonder, when I see extensions and, er, improvements if some people are obsessed with decreasing the value of their property! Plainly  not, but I feel that some don't give as much thought and design attention to detail, and ask questions that they might if building from scratch. The considerations should include assessment of the impact of the extra room (s) on existing daylight, ventilation and circulation, for a start. Stand back and consider the new massing rather than going with your first thoughts, a builders suggestion, or even a neighbour. This is especially true for internal circulation and possibly resultant dead spots.

Consideration can simply be of the choice of materials. Trying to match old and new bricks rarely works, with differing formats, to say nothing of the abilities of a bricklayer achieving thin coursing. Instead consider contrasting materials, (or joining by separation, visually) which can work and is especially dependent on locations and local vernacular...there may well be a local tradition of change (driven by necessity or availability) from days of less ridiculous nimbyism and the "that doesn't fit in brigade" that you can show evidence of to support your desires to a planner, who should instinctively be in your side.

If I can again put in a plea for consideration of 'separated' addition (a pavilion in the rear or side garden) which the builder could get on with without disruption to the existing house. This can joined with a corridor or glazed link, and create, perhaps, an interesting, protected courtyard...quite a case for a picture sparing saving a thousand words. OK it's probably a loss of some garden, yet only lawn anyway. but it's really just another way to look at spatial issues, and discovering fresh advantages. there will no doubt be height issues, with a neighbours blessing you could even build a windowless  wall of the pavilion on a boundary, if space is tight...Throw away the prejudices and think of the benefits and positives...a corrugated card massing model anyone?