The Federal penalty is 10%, your State may or may not have additional penalties. So you would owe taxes on the money as if it were income in the year you made the withdrawl. In addition to the taxes, you would owe an additional 10% (to the Fed).
When you make your withdrawl, your 401K custodial is required to withhold 20%. This is similar to the withholdings made on your paycheck. The 20% isn't specifically for any certain amount, rather in anticipation of what you will owe between taxes and the penalty. Meaning, when you file your taxes for the year of the withdrawl, you may owe the balance (of taxes and penalty).
One way to do what you want is change jobs (or quit) and roll over your 401(k) into an IRA. Then you can "self-direct" your IRA to invest in real estate. There's a great (recent) book on this called IRA Wealth by (I think) Patrick W. Rice. At any rate, I know the title is correct. The book is specifically about using your IRA to invest in real estate.
Here is the catch, if you roll your 401(k) into a SDI (Self-Direct Ira) you will pay taxes if it is a Roth. Most experts suggest investing in real estate through a Roth rather than traditional. Get the book IRA Wealth and look at
http://www.trustetc.com/