What you call a repair is not a repair. It is a property improvement because it meets both requirements of a property improvement as defined by the IRS.
1) It adds "real" value to the property. In other words, when the property is appraised by a certified, qualified licensed property appraiser, they will appraise it at a higher value than they would without the new septic system.
2) The property improvement because "a material part of" the property. In other words, it's not something that you can reasonably and feasibly remove prior to selling the property. But if you did remove it, there is no question that it would reduce the sale price and value of the property.
Finally, rental property does not qualify for the SEC179 deduction. But the program may offer you the option to take the 50% special depreciation allowance. The program has no way of knowing if a property improvement you enter qualifies for this allowance or not. So be aware that this property improvement does not qualify for this allowance and has to be depreciated over 27.5 years.
On a personal note, I can't understand why anyone would even want to take the 50% special depreciation allowance on a residential rental real estate property asset; especially if they have a mortgage on the property. Doing so will not make one penny of difference to you tax liability. But when you sell the property you are required to recapture all prior depreciation and pay taxes on it in the year you sell it. That recapture will increase your AGI and has the potential to put you in a higher tax bracket.
