We tend to tackle these expenses with a mixed approach. First, a $15k project where you are doing most of the work yourself doesn't mean a full $15k outlay up front (or at the end of the job). You'll have to lay in supplies, maybe hire machinery etc but if you're doing the work while working your regular job then realistically the costs will be spread over a number of weeks/months. So some of it you will be able to cash flow from your general expenses. Furthermore, if you are home working on the yard you are not out spending money on travel, food etc. Additionally, you say that you send your spare money each month to investments, rather than investing a fixed amount per month. This will mean that naturally in the months you are spending on the yard, you are sending a bit less to your investments.
If we have a month during the project where we spend a little more than we can manage to cash flow, we would use cash from our EF - as the time on the project passes the interest on the EF will help to build it back up, and you can always divert some investment cash to refill the EF once the project is done. I view my EF not as an absolutely fixed figure, but as a pot with a little flex in it to accommodate our needs. So I don't need exactly $45k every day of the year. If for a few weeks it's at $40k, it's still an EF! If you don't have this mental buffer, you could look at increasing your EF to say $50k before you start the work and then using the buffer. Alternatively, you could use 0% credit cards to finance the supplies/hire elements and pay it off gradually over a year or so. We tend to do this for vehicles and often manage to keep a rolling 0% balance for several years by switching to new cards. We are in the UK though and the way cards work may be slightly different.
Anyway, hope this is helpful. Any approach that spreads the cost over several months rather than an upfront (or final) lump sum will minimise the risks/costs of spending the money, in terms of investment returns, risk of needing the EF, interest on the EF, etc etc.