Inserting a small vertical space in a table

you can load array package then

setlengthextrarowheight{3pt}

the array package is a required part of the core latex distribution, so will be available on all installations.

For a general reference on how to improve the spacing in tabular and array lines, see the article "Correct spacing for tables and arrays" by Claudio Beccari on p. 10 of TeX and TUG News 1993 (Vol. 2, No. 3).

His method, which involves judiciously inserting "struts", applies to lines in tabular (as well as tabular*, supertabular, xtabular, longtable) and array environments which contain

• superscript material, on a line that's preceded by an hline,
• subscript material, on a line that's followed by an hline, and
• any other lines with material (including hlines) above or below them that might result in a cramped look of the output.

He suggested defining a "top strut" and a "bottom strut" as follows:

newcommandT{rule{0pt}{2.6ex}}       % Top strut
newcommandB{rule[-1.2ex]{0pt}{0pt}} % Bottom strut

Using your MWE as a starting point -- by the way, 9pt is not a recognized option in the article document class, so I'm omitting it -- one could put these macros to use as follows:

documentclass[letterpaper]{article}
usepackage{helvet}
renewcommand{familydefault}{sfdefault}

newcommandT{rule{0pt}{2.6ex}}       % Top strut
newcommandB{rule[-1.2ex]{0pt}{0pt}} % Bottom strut

begin{document}  
begin{tabular}{ p{8em}  r  r  l }  
hline      
          & Total     & Average   & Unit TB      hline      
Area 1    & 419773    &   9.15    &  mtextsuperscript{2} T       
Volume 1  & 0         &      0    &  mtextsuperscript{3} B     hline  
end{tabular}

end{document}

The simplest solution was given by David Carlisle in How to add vertical space struts after hline?

hline
noalign{vskip 2mm}    

For those who can use the bigstrut package, then just inserting bigstrut[t] will fix the problem.

documentclass[letterpaper]{article}
usepackage{helvet}
usepackage{bigstrut}
renewcommand{familydefault}{sfdefault}

begin{document}
begin{picture}(0,0)

put(20,-136){mbox{    
        footnotesize   
        begin{tabular}{ p{8em}  r  r  l }  
        hline      
                        & Total     & Average   & Unit   
        hline      
        Area1           & 419773    &   9.15        &  emph{m$^2$} bigstrut[t]       
        Area2           & 0     &   0       &  emph{m$^3$}      
        hline  
        end{tabular}
    }}  

end{picture}

end{document}

fwiw, http://www.tex.ac.uk/cgi-bin/texfaq2html?label=struttab summarises all the above, and mentions a couple of other packages. (apologies for intruding with an obvious pointer.)

It may be worthwhile to take a look at the "booktabs" package, http://www.ctan.org/pkg/booktabs. Then, using e.g. the command midrule instead of hline provides additional space between the columns.

use

    addlinespace[2ex]

in the beginning of a line.

Of the booktabs package (which you want to use anyway to polish up your tables).

rule{0pt}{4ex}} x&x ;x&x ;x&x

I found a cool solution for putting spaces inside a table. Simply add a row containing vspace{-0.3cm}. This will add a row to the table and then back up the spacing by 0.3cm or whatever you want.

It’s easy to set rowsep with tblr environment of the new LaTeX3 package tabularray:

documentclass[9pt,letterpaper]{article}

usepackage{helvet}
renewcommand{familydefault}{sfdefault}

usepackage{tabularray}
SetTblrInner{rowsep=2pt}

begin{document}

begin{picture}(0,0)
put(20,-136){mbox{    
        footnotesize   
        begin{tblr}{ p{8em}  r  r  l }  
        hline      
                        & Total     & Average   & Unit   
        hline      
        Area1           & 419773    &   9.15        &  emph{m$^2$}            
        Area2           & 0     &   0       &  emph{m$^3$}      
        hline  
        end{tblr}
    }}  
end{picture}

end{document}