<PAGE>
 
                                EXHIBIT 11.1

                      BUILDING ONE SERVICES CORPORATION
                     COMPUTATION OF NET INCOME PER SHARE
               (Dollars in thousands, except per share amounts)

<TABLE> 
<CAPTION> 
                                                                     Three Months Ended                Nine Months Ended
                                                                       September 30,                    September 30,
                                                                    1998            1997             1998           1997
                                                                -------------   -------------    -------------   ------------- 
<S>                                                             <C>             <C>              <C>             <C>  
Basic earnings per share:                                     
   Net income................................................    $    16,639     $     1,608      $    35,389     $     4,622
   Weighted average shares outstanding - Basic...............     44,159,551       6,562,035       40,655,279       6,125,692
                                                                 -----------     -----------      -----------     ----------- 
   Net income per share - Basic..............................    $      0.38     $      0.25      $      0.87     $      0.75
                                                                 ===========     ===========      ===========     =========== 
Diluted earnings per share:                                   
   Net income................................................    $    16,639     $     1,608      $    35,389     $     4,622
                                                                 -----------     -----------      -----------     ----------- 
   Weighted average shares outstanding - Basic...............     44,159,551       6,562,035       40,655,279       6,125,692
   Convertible Non-Voting Common Stock.......................        500,000                          500,000          65,934
   Common stock equivalents from stock options and warrants..        192,711         420,885          524,258         354,951
   Contingently issuable shares..............................        440,852                          180,679
                                                                 -----------     -----------      -----------     ----------- 
   Total weighted average shares outstanding - Diluted.......     45,293,114       6,982,920       41,860,216       6,546,577
                                                                 -----------     -----------      -----------     ----------- 
   Net income per share - Diluted............................    $      0.37     $      0.23      $      0.85     $      0.71
                                                                 ===========     ===========      ===========     =========== 
</TABLE> 


Outstanding stock options and warrants to purchase 6,255,774 shares of common
stock as of September 30, 1998 were not included in the computation of diluted
earnings per share because the options' exercise prices were greater than the
average market price of the common shares during the period.