I came around this problem (depends on SQL Server 2008 R2 jutst to SQL Server 2012) by reading Paul S. Randal's and Ana's blog posts:

• How does DBCC CHECKDB WITH ESTIMATEONLY work (Paul)
• DBCC CHECKDB WITH ESTIMATEONLY: Do you trust it? (Ana)

DBCC CHECKDB runs CHECKALLOC, CHECKTABLE, CHECKCATALOG etc. against database, tables, views and so on. And I have learnded, however CHECKDB is executed against master database, a second CHECKDB is also running internally on the mssqlsystemressource.

So in my way, I want only make consistency check of disk space allocation by DBCC CHECKALLOC WITH ESTIMATEONLY against a ~ 200GB database, to know the estimated amount of tempdb space.

I run following DBCC command on SQL Server 2012 Enterprise Edition:

DBCC CHECKALLOC () WITH ESTIMATEONLY, TABLERESULTS;
GO

And I get this result

Estimated TEMPDB space (in KB) needed for CHECKALLOC on database XXX = 35383596.

OK, nothing special - but I thought about the posts of Ana and Paul, and a blog entry of Running SQP on SQL Server Blog - Faster DBCC CHECKDB Released in SQL 2008 R2 SP1 CU4 : Traceflag 2562 & 2549

So next shot comparing CHECKDB and CHECKALLOC:

DBCC CHECKDB WITH ESTIMATEONLY, TABLERESULTS; 
GO

DBCC CHECKALLOC WITH ESTIMATEONLY, TABLERESULTS;
GO

And the results are diffrent!
For CHECKDB:

Estimated TEMPDB space (in KB) needed for CHECKDB on database XXX = 27165.

And for CHECKALLOC:

Estimated TEMPDB space (in KB) needed for CHECKALLOC on database XXX = 35383596.

This isn't an expected result - what a discrepancy!

By the way I have made a Microsoft Connect entry: http://connect.microsoft.com/SQLServer/feedback/details/787049/discrepancy-by-estimated-tempdb-space-for-checkdb-and-checkalloc-with-estimateonly

So feel free to vote.

I wish you a nice day,
tosc