How to run SELECT queries in PL/pgSQL IF statements

I am trying to run SELECT queries in PL/pgSQL IF statements using the code below:

DO
$do$
DECLARE
    query_type   real;
    arr real[] := array[1];
BEGIN
    IF query_type = 1 THEN
        RETURN QUERY
        SELECT "Westminster".*
        FROM "Westminster"
        WHERE ("Westminster".intersects = false AND "Westminster".area <= 100);
    ELSE IF query_type = 0 THEN 
        RETURN QUERY
        SELECT "Westminster".*
        FROM "Westminster";
    END IF;
END
$do$

However I get the following error, ERROR: cannot use RETURN QUERY in a non-SETOF function.

Does anyone know how I can get the above code to work? Thank you.