Parents' house near new Amazon HQ, please review my plan

[Sergio Santos]

Hi there, thanks for reading!

So here's the deal:

DETAILS

• Mom & Dad own house, they're now retired and living mostly overseas. 
• House in Arlington, VA...not far from Crystal City (amAzOn)
• 2nd smallest house on the block, 3br, 2ba Worth ~$700k (most houses on block ~$900k)
• 1st smallest house on block next door...worth ~$680k
• So, to own both properties:

• Spend: ~$680k+ 
• Total Value: ~$1.38m

PLAN

1. Attempt to buy neighbors house.
2. Demolish both houses and build a 4 unit townhome (see pic for example).
3. 3 others to rent, 1 for family

QUESTIONS

Build Costs - Anyone have insights into building costs for a 4 unit townhome building in Arlington? And any other significant costs to be aware of?  Understand there's a ton to consider here, just  looking for ballpark estimate for now.

Financing - We have about $50k of cash and great credit between my parents and I. With the cost of neighbor house (~$680k) plus build costs for the 4 unit townhome ($??), plus other expenses.  Just wondering out loud if we can expect to get a bank loan for all costs, or should expect to raise some private. 

Beyond that, any other thoughts, suggestions really appreciated!

Thanks again for reading!