BODY FAT PRECENTAGE

Yes, if you gain muscle, your BMI goes up. That's why BMI isn't considered a very good measure for a healthy weight. It's meant to be a tool to measure averages among people groups. Individuals who are particularly muscular will find themselves with a BMI that says "overweight" even though they may have a very low body fat percentage.

The best measure of body fat % is a DEXA scan, a bodpod (hydrostatic weighing), or good old calipers. You can use some of the formulas on the Fat2Fit Radio site too: http://www.fat2fitradio.com/tools/

BMI chart is just a tool to measure your weight vs your height & while it is fairly accurate for the general population but it is not the end all be all when it comes to determining whether you're healthy or not. It also just tell the total bodyweight but it doesn't take into account how much of it comes from lean body mass (muscles, bones, organs, water) & fat. In short, it doesn't tell your body fat percentage.

For you to determine your body fat percentage, you either have to use calipers, online calculators using your body measurements, BIA scales or the more accurate ones which are DEXA scans or hydrostatic water weighing in.

As for muscle gain, well usually your weight does go up but your size either stays the same or goes down. However if you're on a calorie deficit, unless you're a beginner or morbidly obese, it is impossible to gain muscle. You have to eat in surplus to gain muscle. But it doesn't mean that you'll not going to be stronger in a deficit. When you lift weights, you also strengthen your bones & make them more dense hence it will weigh a bit heavier than before plus the temporary water retention that our muscles are holding on to after a workout session.