Learn Spanish (Spain): ¿Conoces algún piso exterior que sea soleado? - Do you know any outward-facing apartment that is sunny?

The verb sea is in the subjunctive mood because the speaker doesn't know if this sunny apartment actually exists. When a relative clause (starting with que) describes an indefinite, unknown, or hypothetical thing like algún piso (any apartment), Spanish requires the subjunctive. If they were talking about a specific apartment they already knew existed, they would use the indicative es.

What exactly does piso exterior mean in this context?

In Spain, it is common to distinguish between a piso exterior (an apartment facing the street) and a piso interior (an apartment facing the building's inner courtyard). An exterior flat typically gets much more natural light, which connects to why the speaker wants it to be soleado (sunny).

Why does algún have an accent mark here?

Algún is the shortened form of alguno (any). It drops the final -o because it comes directly before a masculine singular noun (piso). The written accent mark is added to maintain the spoken stress on the final syllable, keeping the emphasis exactly where it is in alguno.

Why isn't there a 'personal a' after conoces?

While you frequently see the 'personal a' after conocer when asking if someone knows a person or a pet (e.g., ¿Conoces a María?), you do not use it for inanimate objects. Since algún piso is a place rather than a person, the 'personal a' is left out.