Indeed, there are relatively restrictive laws on constructing buildings nearer than 100 metres from the sea or a lake. Given that Sweden has a lot of coast and bushels of lakes, there is a lot of beachside to be protected.
det can be either a pronoun or an article, but not an adverb.
In a presenting construction like we have here (corresponding to 'there is' in English), it's a pronoun.
In den gröna boken 'the green book', it's an article.