box culvert not done means need approvals from BCA, LTA, PUB, NParks.
i also had to do up the box culvert for my house along with all the necessary works like new drainage, railings, kerb, etc. cost me more than 10k just for this part in construction cost.
for the box culvert, what you need to do is just what is required for your entrance. don't have to do up the entire width of the house. no point you spending money to do up public infrastructure.
yes, approvals can take 3 to 6 months because of all the waiting time in between and there's so many agencies to clear. and you need to clear the agencies like LTA, PUB first before the final plans can be submitted to URA and BCA for approval and then issue of permit to start works.