SQL - How's my formatting?
What you've got is pretty readable, but there are a few points to make:


Your sub-query is written with a different indentation style from the main query.
I prefer to see the keywords in upper-case.
Personally, I dislike spaces before commas intensely (and you aren't 100% consistent about adding them).  However, I loathe even more the style where the comma is put at the start of the next line:

     item1
   , item2
   , item3

I'd use explicit AS to introduce table aliases.  At least on the DBMS I use mainly, using AS gives a degree of future-proofing against aliases that become keywords in a later version of the product.  That is, writing FROM SomeTable st runs into a problem later if st becomes a keyword, but writing FROM SomeTable AS st avoids that problem.  Whether this helps in other DBMS is open to debate.


For your query, I'd probably write:

SELECT lp.period_id,
       lp.building_id,
       lp.suite_id,
       lp.lease_id,
       lp.tenant_trading_name,
       lp.suite_status,
       lp.suite_name,
       lp.suite_area,
       lp.lease_status,
       lp.lease_current_start_date,
       lp.lease_current_stop_date,
       lp.lease_current_term,
       lp.lease_occupancy_date,
       lp.lease_vacated_date
  FROM property.lease_period AS lp
 WHERE lp.lease_current_stop_date &lt; @truncateddate 
   AND (UPPER(lp.lease_status) = 'ACTIVE' OR UPPER(lp.lease_status) = 'OVERHOLDING')
   AND lp.building_id = @building_id
   AND NOT EXISTS
       (SELECT *
          FROM lease_deal.lease AS l
         WHERE lp.building_id = l.building_id
           AND lp.lease_id = l.lease_id
       )
   AND (@section_name = 'ALL' 
        OR (@section_name &lt;&gt; 'ALL' AND UPPER(section_name) = UPPER(@section_name)))
   AND lp.lease_current_stop_date BETWEEN @truncateddate AND
                        DATEADD(MONTH, @lease_expiry_period, @truncateddate)
 ORDER BY period_id DESC;


Sometimes, I'd indent the sub-query more than I did here, occasionally even placing it after the EXISTS:

   AND NOT EXISTS (SELECT *
                     FROM lease_deal.lease AS l
                    WHERE lp.building_id = l.building_id
                      AND lp.lease_id = l.lease_id
                  )


That has a tendency to run off the right-hand edge of the screen, though.