- Peninei Halakha
Since we curtail our joy from the beginning of Av, one may not build anything that brings joy during the Nine Days. For example, one may not expand one’s house or porch unless there is a vital need for this. In addition, one may not plaster or paint the walls of one’s home, because these are considered luxuries that make one happy, and one can live without them (sa 551:2). Similarly, one may not do renovations that are designed to beautify one’s home, like replacing shutters, closets, curtains, or anything else that is valuable, brings one joy, and is not absolutely necessary.
However, one who lives with his family in a cramped living space may build an additional room during the Nine Days. One may also engage in any type of construction that is designed to prevent damage. For example, if a wall is about to collapse, one may demolish it in an orderly fashion and rebuild it, even if he does not need the room where the wall is located and there is no danger involved, because this prevents damage to his property.
Likewise, one may build, plaster, or paint for the sake of a mitzva, such as building a synagogue or a school (mb 551:12, Kaf Ha-ĥayim 551:25). Arukh Ha-shulĥan (551:7) states that anything that is necessary for the public good is considered “for the sake of a mitzva” and is permissible.
Similarly, one may not plant anything that brings joy, like decorative trees, hedges, or flowers (sa 551:2). However, one may maintain a decorative garden, water the garden, mow the lawn, and continue regular maintenance.
Any planting that has an actual purpose is permissible. Therefore, one may plant fruit-bearing trees during the Nine Days. Similarly, one who makes a living growing decorative bushes and flowers may plant them in his nursery in order to sell them.
 Building or repairing non-essential shutters and closets is considered joyous building. Thus states Igrot Moshe, oĥ 3:82, regarding a closet. It also seems proper to forbid putting up decorative wallpaper, as Be-tzel Ha-ĥokhma 4:54 states (and contrary to Igrot Moshe loc. cit.). Similarly, one may not install curtains that are purely ornamental; thus states Hilkhot Ĥag Be-ĥag 4:4. If, however, the main purpose of installing curtains or shutters is to ensure privacy, it would seem that it is permissible. The example of joyous building given in Ta’anit 14b is a “matrimonial house” for one’s son. This is not relevant today, which is why I did not mention it in the main text.