How to get an estimate for your ADU?

Why is it so difficult to get an ADU estimate over the phone or on the spot? See below what builders have to look at before they give you an estimate, and even then it will be an estimate, not a firm proposal. The latter needs some involved calculations, that no one will be able to give you before some field investigations by the crew. And frankly, the time spent on it should be paid for. Just look at this puzzle below. Every item is a variable that influences the estimate.

Access to utilities

This is the elephant in the room. How far do you need to trench, how good the condition of laterals is.

Overhead utility lines

These can be a real nuisance during construction, and prevent use of cranes and modular units.


Just one photo like this often adds $3500 to the estimate – the panel upgrade is required.

Water and sewage line for the new unit

It had happened before – the sewage line had to be updated to accept the additional volume from the new unit. Existing clean outs are a good indicators.

Photo by Vivint Solar on
Solar panels

Does the main house have solar panels installed? Does it have reserve capacity? As of January 2020 any new construction requires installation of the solar panels.

Yard access

This determines construction method. Wide access – you can bring in a modular unit. Narrow access – we will have to figure out if we need to use traditional methods, wall panels or a crane to bring the unit in.

Fire hydrant

If you are close to it – wonderful. If not as close – then how far exactly? Depending on your municipality, this might trigger an installation of a new fire hydrant or in-unit fire sprinklers.

Sub-panels inside the house

For attached units the sub panels also play a role.

Foundation issues

Planning a garage conversion? Is it level? Does it have cracks? This will be a red flag for the contractor to build in some contingency budget for the foundation work.