How to interpret the meaning when someone tells me that a particular building is down the street? Is it at the end of the road, middle or at the opening?
You will hear "up the street" as well as "down the street". Both essentially mean "along the street", and are generally from the perspective of the speaker - that is whether they consider the direction to be "up" or "down". It does not refer to a specific location on the street such as the end or half-way along, but to a direction away from where you already are. If we mean to indicate that something is right at the end we will say so, using either "at the bottom of the street" or "at the end of the street".
In British English, we tend to use "up" and "down" when speaking about travelling from one part of the country to another - "up" when travelling broadly in a northerly direction, and "down" when travelling southwards, for example:
I'm going down to London.
I'm going up to Glasgow.
When it comes to talking about travelling along a street I doubt very much whether many people give much thought to which compass direction they are facing, so in answering this question I did look to see how often "up the street" is used versus "down the street", and as this ngram shows, "down the street" is used far more. I would suggest that the reason for this is, unlike when compass direction is an influencing factor, "down the street" far better describes travelling away from a fixed point from the perspective of someone giving directions.
As an aside, the expression "up my street" is a saying used idiomatically to mean that something is familiar or well-suited to a person, or in their field of expertise, which may explain why this ngram shows it can be used more than the equivalent expression that uses "down".