all the diets that I seem to have come across are either based around losing fat and weight, or building muscle and gaining weight.

are there any diets which allow for both to be done?

Also, should a diet plan be altered to allow for more proteins / fats before workout and more carbs after workout? or just left as it is?

this is because Im most situations you can only gain weight or loose weight. now there is somthing called beginner gains where you can loose fat, and gain muscle at the same time. In a nutshell why you can't have bolth is because if you want to gain muscle you need HIGH calories, and to loose fat you need LOW calories. you want protein post workout (IS:whey), and somthing that will digest slowly before you go to sleep (IE: cottage cheese). good day to you sir.

Do a search for Dr. Fred Hatfield's Zig Zag Diet.

so basically I need to do one or the other, or one first and then the other?

I haven't checked out that site yet but I'm guessing that I need to first go on a fat losing diet, and then switch to a muscle building diet?

Exactly. You have to figure out what your goals are, adn their importance. A lot of people alternate every few months with fat loss/muscle building.

Like Phaledox said, to lose fat, you have to eat less and do more cardio - however, those two things will also help you lose muscle. And you can't lose fat while on a muscle-gaining diet.

Technically, you can take on a diet somewhere in the middle...where your physical activity will burn fat, slowly, while still allowing for muscle gains...also slowly. But if you lean hard one way or the other, you'll see faster muscle gains or faster fat loss.

On fat loss diets, you need to keep the protien intake high to prevent muscle loss. On muscle building diets, you tend to increase carb intake as this promotes anabolism and ensures you have a caloric surplus, triggering your body's response to build muscle due to the abundence of nutrients, and the increased physical workload of your workouts.

a pretty decent book/training routine is Body RX by A Scott Connelly. It has 3 6week intervals with different training and diets. you start by massing up then you tone up a little, then you cut the fat. There's a 4th 6week cycle for maintenance, or you can just repeat 1-3 over again until you get your body sculpted how you want.