In most cases, it will not cause the birds any harm if you look into the birdhouse, but all the same, don’t make a habit of it. If you do check on the birds, only stay for a few seconds and gently knock on the birdhouse beforehand to let them know you are coming. If you see babies, stay away for a couple of weeks. Even though you built the house, let nature run its course as much as possible.
You do not have to clean out your nest, as some birds might prefer old nesting material, but in all likelihood, it won’t hurt to clean out your nest either. Try both methods and see if the birds in your area seem to prefer a clean birdhouse or one with nesting material in it.
If you are going to clean your birdhouse, wait until the fall, and after you are sure that your residents have moved out for the winter. Hopefully, your birdhouse has a removable top or bottom or a hinged door that you can open. In any case, you need to empty out your birdhouse. Lightly brush the sides clean as well, and if you have wasp or insect problems on the ceiling, rub it with a bar of soap. Leave the birdhouse open for a day or so to let it completely air out and dry out. For the most part, you will only need to clean out your birdhouse once a year, but glance at the house again in the late winter to make sure it is ready for the next breeding season.
Most birdhouses are made of wood, so you will not need to clean the exterior. If you have painted your house with non-toxic paint, touch up the paint as needed. If your house has shingles or another non-wood substance covering a portion of the exterior, take the house inside and use detergent and mild bleach to clean as needed. Rinse the exterior and then let it dry completely before returning it to its original location.