Here we go....
If you are going to make this conversion (and you want to do it right) you are going to want to chheck with your city to see what upgrades you may have to do. this can be costly....but very possible. My question is...have you thought about the cash outlay this may require?
As far as the comps go...2006 will not matter. Different market...different prices. As a matter of fact, when people use the ole..."Well in 200X it was worth X", it's usually because they know the value has dropped...often times pretty significantly..especially in the higher priced properties. Now there is a possibility this is not the case at all for your property. Just make sure to get recent comps.
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If you are getting a loan, the bank will make sure it will appraise anyway.
Now here is the VERY IMPORTANT thing...
when you take it from a residential property you will using the Comps or comparable approach to determine the value...so if the other "mansions" of similar size are selling for 500K...then arguably yours is worth 500K etc.
When you convert this bad boy....you will be using the Income approach to determine the value of the property. So you want to make sure you aren't going to decrease the value by doing the conversion. You can google the income approach...or I may have written about it on my blog. I am not sure...either way, learn about it!
Lastly, there is a saying in this industry..."What's it worth is not the same as What's it worth to me?".
The reason I mention this is because if you are making this investment...it should be for a purpose....to keep in allignment with a goal of some sort....a step towards an end in mind. So for example if your goal is 10K/mo cash flow from properties and the purchase and conversion will allow you to achieve this...then your answer is obvious.
Here is another example: Your goal may be to pull outa quick 200K and still cashflow the property 5K and maybe you can accomplish this by buying it for 500K using the comparable approach; however, after you do the conversion you refinance it using the income approach and it's worth 1M and you get a loan for 700K. Now you paid off the original note for 500K...made 200K and hopefully the expenses are lower than the income so that you have your cashflow.
I AM NOT SAYING YOU CAN DO THIS WITH YOUR PROPERTY
becasue I do not know your numbers; however, these things are possible if your numbers work. Run the numbers and weigh your options!
Hope that helped!
Amigos,
Mark