Engineeered retaining wall - ~10 feet high
We are about to purchase a house and requested an estimation on extra work required from the builder. The house, as is, has very small yard and we wish to have a bigger yard, leveled with the current small yard, and plan to use an engineered retaining wall in order to achieve this goal. The builder has plenty of excavated land/gravel to fill in the planned yard, basically from a neighbour lot.
We don't have the exact measurements , but based on the lot size - the wall will be around 100 feet long, and 10 feet high, more or less.
The estimation was shared with us and was also added to the overall house price. It is $35000, and seem outrageously high to us. The sales manager claims this is a risk, since the actual estimation might be higher, and he will also need to pay his engineer around 3K for certification.
Since additional lots have the same problem, the builder has done it before. I am attaching a picture similar to what the neighbours have in place.
Please help me to understand if the price is correct, and how complicated is it to do it ourselves after purchase - the permits, the fill, and the headache.
Really appreciate your help, Thank you!