6.2 Basis of Design, Alternatives Evaluation & the Preferred Scheme
Key Takeaways
- The Basis of Design records the criteria, assumptions, standards, and performance targets a design must satisfy, and is the document against which alternatives are evaluated and later changes are measured.
- Design alternatives must be genuinely distinct in organizing concept, not variations in furniture layout, or the evaluation produces a predetermined result.
- Evaluation criteria and their weights must be fixed and agreed before alternatives are scored, because weights assigned after scoring simply ratify a preference.
- A weighted evaluation matrix documents the reasoning behind a selection; its value is the recorded rationale, not the arithmetic total.
- Refinement of the preferred alternative should incorporate the strongest features of the rejected schemes rather than discarding them wholesale.
1. Three Tasks, One Workflow
The Schematic Design content area carries 28% of the Planning and Design section and includes three sequential tasks that the exam treats as one professional workflow:
- Create the Basis for Design — fix the criteria before drawing
- Evaluate Design Alternatives — test distinct schemes against those criteria
- Refine Selected Alternatives — develop the preferred scheme and absorb the best of the rejects
Skipping the first step is what makes the second step meaningless.
2. The Basis of Design Document
The Basis of Design (BOD) is the written record of what the design must achieve and the assumptions on which it rests. It is distinct from the program: the program states what (quantities, uses, areas); the BOD states how well and under what constraints.
A landscape BOD typically records:
| Category | Content |
|---|---|
| Performance criteria | Stormwater release rate and water quality volume, accessible route standards, illumination levels, capacity targets, maintenance level of service |
| Applicable standards | Adopted codes, ADA/PROWAG, CPSC and ASTM for play, NPDES permit conditions, agency design manuals with edition dates |
| Design assumptions | Assumed soil bearing, seasonal high water table, design storm, design vehicle, projected user counts |
| Owner requirements | Operations and maintenance capability, budget, phasing, durability and life expectancy |
| Site constraints carried forward | Absolute constraints from the analysis synthesis, easements, prior commitments |
Why it matters later. During construction administration, an RFI or substitution request is evaluated against the Basis of Design. When the BOD states "planting soil shall achieve an infiltration rate of not less than 2 inches per hour," a substitution proposing a heavier media has a documented standard to fail against. Without a BOD, the argument becomes a matter of taste.
The BOD is also the instrument that makes design assumptions falsifiable. Stating "assumed seasonal high water table at 48 inches, to be confirmed by geotechnical investigation" converts a hidden assumption into a tracked verification item.
3. Generating Genuinely Distinct Alternatives
The most common failure in alternatives evaluation is the straw man: presenting one developed scheme and two deliberately weak options so that the preferred scheme wins. Reviewers and clients recognize this, and it destroys the credibility of the process.
Distinct alternatives differ in organizing concept, not in detail:
| Genuinely distinct | Not distinct |
|---|---|
| Concentrated development on the upland with the lowland preserved, versus dispersed development across both | The same plan with the parking rotated |
| Vehicular access from the north arterial versus from the south local street | The same access with a different curb radius |
| Stormwater managed in a single regional basin versus distributed LID cells | The same basin with different planting |
| Program organized around a central commons versus along a linear spine | The same commons with different paving |
Each alternative must be developed to comparable resolution. Comparing a fully studied scheme against two sketches is a straw man by another route.
4. Evaluation: Criteria, Weights, and the Matrix
The sequence that makes evaluation honest
- Define criteria from the Basis of Design and the program goals
- Assign weights reflecting relative importance, with the client and stakeholders
- Lock criteria and weights before scoring
- Score each alternative against each criterion, with a written rationale for each score
- Compute and review, then interrogate the result
Weights must precede scoring. Assigning weights after the scores are known allows the evaluator to tune the weights until the preferred scheme wins, which converts an analytical tool into a justification device.
A worked structure
| Criterion | Weight | Alt A raw | Alt A wtd | Alt B raw | Alt B wtd | Alt C raw | Alt C wtd |
|---|---|---|---|---|---|---|---|
| Habitat and tree canopy preservation | 25% | 3 | 0.75 | 5 | 1.25 | 4 | 1.00 |
| Capital cost | 20% | 5 | 1.00 | 2 | 0.40 | 3 | 0.60 |
| Accessibility and connectivity | 20% | 4 | 0.80 | 3 | 0.60 | 5 | 1.00 |
| Stormwater performance | 15% | 2 | 0.30 | 5 | 0.75 | 4 | 0.60 |
| Operations and maintenance burden | 10% | 4 | 0.40 | 2 | 0.20 | 3 | 0.30 |
| Phasing flexibility | 10% | 3 | 0.30 | 3 | 0.30 | 5 | 0.50 |
| Total | 100% | 3.55 | 3.50 | 4.00 |
Alternative C scores highest, but the matrix's real output is the pattern: A is cheap and easy to maintain but performs poorly ecologically; B is the ecological performer at the highest cost; C is balanced and the most phasable. That pattern, not the decimal, is what the client decides on.
Read the matrix, do not obey it. Scores within roughly 10% of each other are not meaningfully different given the subjectivity of raw scoring. When totals are that close, the decision should be made on the criterion the client cares about most, and that reasoning should be recorded.
Sensitivity testing
A defensible evaluation includes a check: if the weight on the single most contested criterion is raised or lowered by 10 percentage points, does the ranking change? If it does, the selection is weight-driven rather than performance-driven, and that fact must be disclosed.
5. Refining the Selected Alternative
Refinement is not simply drawing the winner at a larger scale. Three moves define it:
- Absorb the best features of the rejected schemes. If Alternative B's distributed stormwater approach outperformed C's, the refined C should adopt it where compatible. Alternatives are a source of ideas, not a tournament with one survivor.
- Resolve the weaknesses the matrix exposed. The criteria on which the preferred scheme scored lowest are the refinement agenda.
- Test the refined scheme against the Basis of Design again. Refinement introduces change, and change can break a performance criterion that the original scheme satisfied.
The deliverable is a single refined schematic plan with supporting sections, a program confirmation, a preliminary opinion of probable cost, and a written record of why this scheme was selected. That written record is what protects the decision when a new stakeholder joins the project six months later and asks why the other option was not chosen.
6. Exam Traps & Pitfalls
- Scoring before weighting. Weights set after scores are known ratify a preference rather than test one.
- Straw-man alternatives. Options must differ in organizing concept and be developed to comparable resolution.
- Treating the matrix total as the decision. Totals within about 10% are not meaningfully different.
- Discarding rejected schemes entirely. Refinement should absorb their strongest features.
- Confusing the program with the Basis of Design. The program states quantities; the BOD states performance criteria and assumptions.
A landscape architect presents three site plan alternatives to a municipal client. Alternative A is fully developed with sections, planting concept, and a cost opinion. Alternatives B and C are single-line sketches with no cost information. The client selects Alternative A. What is the principal professional deficiency in this process?
A landscape architect scores three master plan alternatives against six criteria and then convenes the client to assign weights to the criteria. What is the methodological problem with this sequence?
During construction administration, a contractor submits a substitution request proposing a bioretention media with a documented infiltration rate of 0.75 inches per hour in place of the specified media. Which project document most directly establishes whether the substitution is acceptable?
A weighted evaluation of three park alternatives produces totals of 3.55, 3.50, and 4.00 on a five-point scale. The design team observes that raising the weight on habitat preservation by 10 percentage points would change the ranking. How should this be handled?