Using the WITH CUBE clause at the end of your query will give you an additional row at the end of your result with the sum of the sub_total and all other column values will be null. So in one query you can get everything
SELECT SUM(Project_Total) AS Sub_Total, Project_Code, SectionCode
FROM Temp_Portal
WHERE (Project_Code = ?)
GROUP BY Project_Code, SectionCode
HAVING (Project_Code LIKE 'BA%') AND (SectionCode LIKE 'RN%')
WITH CUBE