Urban Design/Aesthetics Regulations: architecture, billboards, signage etc?

Hello

Im not urban planner or architect byt im involved in a small foundation/group or actvists who are trying to do something about the ugliness and lack of any control on how our city - Warsaw the capital of Poland looks like.

The biggest problem seems to be the fact that anyone can do ANYTHING with property they own i.e install ugly billboards or cover whole XIX century tenements in big printed ads for years. Also the communities of people who own flats in one building are those who decide on which color the renovated facade will be which usually leads to some horrible looking public space.

We have ambitious goal of trying to change the status quo by working with lawyers and local politicians to change the law and control those who can influence public space of the city.
Being the european version of what is called Libertarianism i have hard time supporting even greater government influence over other peoples property but urban planning and urban design are exceptions to free-market doctrine in my book :)

Can you point me to any documnets, articles etc on how different cities, countries and communities use law to control urban design and aesthetics? What is the international practice, rules etc. Im sure there is not need of inventin a wheel and there has to be some established set of rules that local governments could try to interpret and use.

Thank you in advance.