Start with what the existing house can carry.

An addition has to land on the house already there: the wall you open, the roof plane you extend, and the floor level you meet. Those existing pieces decide how cleanly the new room can join, and what you find inside the connection matters as much as the room itself.

The work also happens while the household is still using the house. Ask how daily movement through the home will change, and for how long, before you treat an addition as the simpler path.

1. What is behind the wall the addition will connect to?
2. How will the new roof and floor join the ones already there?
3. How will the household use the house while the addition is underway?

Then ask what a second building asks of the land.

A guest house raises the same questions as a main home: access, utilities, site work and how it relates to the rest of the property. On rural land, those questions come with an order.

In unincorporated Gillespie County, the County Engineer's office reviews development first. The county requires a Development Permit Determination:

"prior to the development of a property, including activities such as new construction, grading, and paving."

Gillespie County requires an OSSF (septic) permit at any acreage, and the Development Permit Determination and a Site Plan with Narrative go to the County Engineer before the septic application.

Water is the other half. The Hill Country Underground Water Conservation District says "all wells in Gillespie County must be registered with the District," and its rules say one well on 10 acres or less can serve "no more than four households for Domestic Use Only" and still be a registered (exempt) well.

1. Would a guest house tie into the existing septic system, or need its own?
2. Will it share the main well, and do you know the well's output?
3. How will it be reached: driveway, parking, path?

Let the use decide, not just the floor area.

Who the space is for changes which rules apply. Family and occasional guests, a parent living there full time, and paid short-term guests are three different projects, even at the same size.

Inside Fredericksburg city limits, renting for less than 30 days needs a city short-term rental permit and inspection, and an accessory unit has its own rules. Outside city limits, including the ETJ, no city short-term rental permit is required. State hotel occupancy tax (6%) applies to short-term rentals inside or outside the city, and local hotel taxes may also apply.

On a historic property, the street matters too. The City of Fredericksburg says "All exterior changes to a Historic property require an approved Certificate of Appropriateness (COA)." That applies whether the change is an addition or a new building.

1. Who will use the space, and will anyone pay to stay there?
2. Is the land inside city limits, in the ETJ, or out in the county?
3. Is the property in the historic district?

In Fredericksburg, we recently finished a home addition, and we are drawing a guest house remodel alongside a main house remodel. Both started with the same questions.