Not listening to parents is bad karma? This is something that I keep hearing from my grandmother that annoys me so much. Although I am of age (26 to be precise), I am still expected to listen to almost every word that my parents say (at least according to my grandmother).

Not to bring personal matters to the site, but I keep arguing that not listening to parents should not be considered ill-treatment of them. They expect me to wear nice clothes, but I don't. (I prefer wearing the same clothes. Who has time for fashion?) They expect me to marry the one they want. They always say that as a Buddhist I should be very obedient. These days when you turn on the TV to listen to some Buddhist advice, monks most of the time talk about parent-child relationships and go on about how children should always listen to their parents.

I am not saying my parents ask me to do anything bad, but as children we know (sometimes, not always) what is best for us. But when we do what we want all hell breaks loose. Daddy shouting, Mommy crying, and Granny shouting that my bad karma will eventually give me disobedient children just like me.

Jokes aside, I want a serious explanation when it comes to parent-child relationships. I am quite aware of the Sigalovada Sutra, but when we do something we want and parents get hurt, is it always bad kamma? It can't be, right? Otherwise the amount of bad karma Prince Siddhartha would have gained after all the pain his father went through...

Please share your thoughts.
This is something I keep hearing from my grandmother that keeps annoying me so much. Although I am well off age (26 to be precise) I am still expected to listen to almost every word that my parents say (at least according to my grandmother). Not to bring in any personal matters into the site but I keep arguing that not listening to parents should not be considered as ill-treating them.

Not listening is not necessary ill treating but if you are intentionally hurting them then it is negative Karma.
Any way it is a good idea to listen to your parents but it does not mean you have to do exactly. They may have experiences you might not have. So best cause of action is to get their input and them proactively take the best possible and rational action. Sometimes by just listening you parent maybe happy.
Also be diplomatic when when you have a difference of opinion.

They expect me to wear nice clothes but I don't (I prefer wearing the same clothes. Who has time for fashion?).

Fashion is actually changing and attitudes. Getting too attached or averse to them is not skillful. Best is to get into a happy medium. If you are scruffy then your social acceptance will also go down hence there is some point in having to dress nicely. Everyone has to depend of society even monks to get their requisites hence you should be reasonably acceptable and presentable.

They expect me to marry the one they want.

If you have someone see how you can present the person of your choice. Sometimes there may be issues with your choice you might have not considered. If it is purely driven by lust it might not work. So think about compatibility. Play it tactfully.
Also the following answer maybe of some help: https://buddhism.stackexchange.com/a/14207/295 in choosing the right person.

They always say that as Buddhist I should be very obedient and these days when your turn the TV to listen to some Buddhist advice, monks most of the time talk about parent-child relationship and goes on about how children should always listen to parents.

As mentioned above getting wider range of input is beneficial to make optimal decisions. More than just robotically doing what you parents say what is needed is to make the relationship more cordial. This maybe difficult due to the generation gap.

I am not saying my parents ask me to do anything bad. But as children we know (sometimes not always) what is best for us.

Life choices perhaps may not be determinable until it plays out when you do not know who is right. All you can do make decision with the best facts you have.

But when we do what we want all hell breaks lose, daddy shouting mommy crying and granny shouting my bad karma will eventually give me disobedient children just like me.

Disobedience and rebellious behaviour sometimes maybe rooted in version. What you have to is to see if there is aversion or some sort of irritation is present and this is influencing your cause of action.
If 3 people feel your are in the wrong perhaps take a step back and see if actually what you have done is the best and what other alternatives would have been there. I am not saying you might be wrong but when this happens it is good through revisit something you have done just to reaffirm what you did is best with the information you had.
In addition willfully making your parents happy would be positive karma. So if you can do something to make them happy go for it. It is some time very little things which go a long way.

Jokes aside, I want a serious explanation when it comes to parent-child relationship. I am quite aware of the singalovada sutta but when we do something we want and parents get hurt is it always bad karma?
It can't be right?

If you do it with intention to hurt or your are due to being internally angry then it will be. So try to get into knowing your true intentions and motivation and what it is rooted in.

Otherwise the amount of bad kamma prince Siddhartha would have gained after all the pain his father went through... Pleas share your thoughts.

Prince Siddhartha left the householder's life with the intention of liberation and helping mankind. This is positive. Though incidentally his father maybe hurt it was not his intention.
If you can think of ways to do something without hurting someone then this would be the best cause of action. But with many lifes choices not everyone will be always agreeable and satisfied, hence try to be accommodating as possible within reason and follow through with the best action taking into account to all opinions and parties affected. Keep in check there is no negative motivations, volition of roots in what you are doing.