Is this a question of how well you get along with the in-laws (in my case "out"laws).
Your wish to do the job for free may be a problem because they might feel themselves imposing on you.
Is this something that requires daily work over a short period or can be done over time.
You could go to them with a plan that for XX dollars including your labor, "I can do such and such". This might ease the imposition problem. Lay out a plan with equipment and see what they say.
If this is something that if you screw up, you'll never live it down, then make your recommendations on the submitted plan and walk away from it.
