Short answer
Ask for two options based on the same rooms, finish level and budget boundaries. A partial repair and a complete replacement rarely include the same work.
Find what can stay
Have the appropriate design and engineering professionals assess what can safely be retained. An old plan or an exterior photograph cannot establish structural condition.
Compare like with like
Use one sheet with two columns. Record what each option assumes instead of treating an unknown as a saving.
- Existing structure and utilities to retain or replace
- What changes in floor area, daylight and access
- Temporary housing, investigation and site work allowances
Decide what needs investigation
If the outcome depends on one wall, one foundation or one approval, ask what investigation would resolve it before paying for two complete designs.
Put it into practice.
Compare remodeling and rebuilding
Put the same family priorities next to both options. Ask what can be retained, what needs professional assessment and what each approach would change about daily life.