I would like to represent a bit more conservative point-of-view in this discussion:
If you really want to leverage your stock investment by taking a mortgage on your house, you should be very, very clear about the associated risks.
Yes, the stock market has, on average, quite high returns. However, average returns are not your reality, the sequence of returns in your individual investment period are your reality. Unfortunately, no one can predict the sequence of return for you.
In this case, your reality depends on two factors - housing prices as well as the stock market. At the end of the 7-year period, a lot of potential scenarios are possible that fall somewhere in between the best and worst case:
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Best case: THe stock market and the housing market continue to rise - Your investments increase substantially and you can pay back your mortgage easily with your house sale
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Worst case: The stock market and housing prices plummet - Your investments are down by 50%, your house sale does not even net the remaining principal on your mortgage and you have to delay your FIRE plans
You should also note that
7 years is quite a short time frame for any investment in the stock market. Usually, the longer you are invested, the higher is your chance to reach average returns. For me personally, the heat map on portfoliocharts.com helped me a lot to understand the factor the duration of the investment period has on my expected returns. See the graph on
https://portfoliocharts.com/portfolio/total-stock-market/ Blue boxes show investment periods with positive real returns, while red boxes show negative real returns.
With an investment period of 7 years, 12 of 39 analyzed 7-year-periods had negative or zero real returns! Only with an investment period of 13 years or more, there were no periods with negative real returns.
(Please note, the graph shows real returns and includes inflation, which is a more conservative view, as your mortgage is in nominal dollars. However, you still need to factor in the interest of your mortgage, and in most of the post-1970s time periods, average inflation should have been lower or equal than a mortgage rate of ~3.5-4% - thus the real returns could still be a rough indicator for your excess returns after mortgage interest)
In the end, it is a simple question of whether you are able to sleep well at night with the associated risks of leveraging, or not.
For me personally, the associated risks would definitely outweigh the potential rewards of leveraged investments in this case.
By the way: An alternative to a mortgage could be also a leveraged ETFs or funds on a broad stock index (some examples can be found here: http://www.investopedia.com/articles/etfs/top-leveraged-sp-500-etfs/). At least these ETF do not require your house as collateral and buying them regularly with a part of your monthly surplus could reduce the risk associated to the sequence of returns.EDIT: That was a bit too quick. It is is quite debatable if leveraged ETFs suitable for long-term investments, due the effect of volatility on the leverage. Since these products usually reset the leverage daily, the long term result is unfortunately not 2x or 4x the Index, but often way lower. This article explains the underlying mechanics quite well:
http://www.investopedia.com/articles/mutualfund/07/leveraged_etfs.asp and this one has the full dose of math
http://ddnum.com/articles/leveragedETFs.phpLeveraging your investment in the medium or long term is still possible, but might require different products (e.g., derivatives) and should be done by experienced investors only.