How are you going to run the lines from the garage to the house?

Ionized ..That is correct. We have an attic in the house. Why is a tankless more expensive? Isn't it more expensive to keep a large tank of water hot all the time? Please help me understand.

We will be working with structural engineer on the structure safety for attic installation but would be great to get advice from this forum on issues we have to consider.

Go to the DOE web site and look at their worksheet for water heating energy efficiency. Plug in your current fuel costs. Tankless water heaters are very expensive to buy. They are so expensive that it takes decades to get back the investment.

"Isn't it more expensive to keep a large tank of water hot all the time?"

Depending in how much insulation is on the tank and what environment it is located in the standby losses may not be enough to recover the additional costs over the life of the tankless unit.

This is a question that really has too many variables to be able to adequately give an answer online.

First off, you need to contact your local AHJ to see if they will even permit an attic installation. (Generally in regions subject to freezing they will not permit attic installations)

When a water heater is installed in an attic you are required to have an attic service access (hatch or doorway) within 20' of the water heater location and the hatchway must be large enough to permit passing the equipment through the hatch in the event replacement is necessary.

You must have an approved floor at the installation site that is big enough to support the equipment with an additional 2' of work space on all four sides of the equipment. The floor must have a certified load capacity to support both the equimpent and the weight of a workman and his tools. (Usually each jurisdiction has their own floor load capacity specs)

You must have a 2' wide floored walkway from the access hatch to the equipment location.

(The question here is whether your attic trusses can support the additional weight)

You must have a light fixture in the near proximity of the equipment installation and that light must be controlled by a switch that is located within 6' of the attic access hatchway.

You must have a 125v electrical outlet in the near proximity of the equipment to permit operation electrical tools during installation or maintenance.

You must have a drip pan under all the equipment with a code approved drain.

In the sunbelt it is common to install the water heater in the attic space, but keep in mind that in most instances water heaters are nearly maintenance free. On the other hand, water treatment equipment requires repeated servicing and I doubt if the AHJ would permit is. (The reasoning being, if it is difficult to access the homeowner may opt to neglect the required service.)

When all things are considered IMHO you would probably find it much cheaper to build a small utility room off the back of the house. You need not have a direct access into the utility room from inside the house, It could have an outside door.

This is a retrofit of an existing structure. One of the greatest concerns is what size of gas service line you have now? In most cases a tankless water heater requires a gas supply equal to or sometimes even greater than your furnace and if you present gas line is critically sized it could require installing a new & larger gas line all the way to the municipal gas suppliers line at the street.