Design Addition

Additions are the bread and butter of most practices and they are lost in two places: the joint, where six different systems have to cross at once, and the trigger, where touching one part of a building turns out to require work on parts nobody was touching.

Workflow

Step 1. Establish what is being done and what is known

Ask, in one message:

1. What is proposed: added area, changed area, changed use, or all three?
2. What proportion of the existing building does the work touch, by area and by value? The jurisdiction's thresholds usually key off one of these.
3. Is the use changing, in whole or in part?
4. Has existing-conditions-survey been run? How much of what you will design against is measured rather than record or assumed?
5. Quote the jurisdiction's existing-building provisions, and say which edition.
6. Is the building occupied through the work? Point at phasing-study.
7. Is the building designated or in a district?

Question 4 decides how much of what follows is provisional. Say so once, clearly, rather than hedging every finding.

Step 2. Settle the relationship before any detail

The decision everything else follows from, and it is usually made by default:

Relationship	What it means	What it demands
Matching	The new reads as the old	Materials that may no longer exist, and a joint that hides
Deferring	Clearly new, quietly subordinate	A restrained language and a legible break
Contrasting	Clearly new, clearly itself	A joint that is designed rather than concealed
Absorbing	The old disappears into the new	Usually the most expensive, and a heritage question if designated

State which and why. Then say what it demands of the joint, because a matching addition and a contrasting one need opposite details at the same place: one hides the seam and the other declares it, and both are harder than a seam nobody thought about.

If the building is designated, say plainly that the relationship is not the design team's to choose alone.

Step 3. Work the six things that cross the joint

Each one crosses at the same place and they interfere with each other.

System	Existing side	New side	How it crosses	Value needed from you

• Structure. Does the new bear on the old, beside it, or independently? Independent is usually simpler and takes room. Differential settlement between old and new is real and is the engineer's. Any new load on existing footings is a question, not an assumption.
• Envelope. The four control layers have to carry from new to old, and the old building's layers are often absent or unknown. Point at design-wall-assembly and air-barrier-trace. This is where additions leak, and it is why the joint is a detail rather than a line.
• Services. Does the new work run off the existing systems, and can they take it? Panel capacity, boiler capacity, drainage falls, water pressure, ventilation. Point at explain-panel-schedule. Each is a question for the engineer and each has a habit of being assumed.
• Levels. Existing floors are not level and rarely match a new datum. State where the difference lands: a threshold, a ramp, a step, or the whole new floor set to meet it. It is an accessibility question as well as a construction one.
• Fire separation. Whether the joint is a required separation, and if the new work is a different construction type or occupancy. Point at design-fire-barrier.
• Finishes and movement. The two structures move differently and any finish running across the joint will crack. Point at design-expansion-joint.

Step 4. Establish the triggers, separately and explicitly

The part that surprises clients and costs the most:

Work through, against your quoted provisions, whether this scope requires work in parts of the building nobody proposed to touch:

• Accessibility upgrades to the existing building, its entrance, its route, its toilets.
• Egress: whether the existing exits still serve, and whether adding occupants changes the count.
• Fire protection: sprinklers, alarm, separations in the existing building.
• Energy: whether existing envelope or systems have to be brought up.
• Structural: seismic or lateral upgrades triggered by a threshold.
• Anything triggered by the change of use rather than by the construction.

For each, record the provision quoted, whether it is triggered, and what it would cost the scope. Say plainly when a trigger has not been confirmed with the authority, and recommend asking before the scheme is fixed rather than at permit. Point at permit-expeditor and senior-code-consultant.

A trigger found at permit is a redesign. A trigger found now is a budget line.

Step 5. Report

• How much of the design basis is measured versus record or assumed, first.
• The relationship chosen and what it demands.
• The six-system table with every open value counted.
• The trigger table, with those unconfirmed with the authority flagged.
• What has to reach the structural engineer and the MEP engineer.
• What should be asked of the authority before the scheme is fixed.

Close by saying that nothing was recalled, and that the triggers are the jurisdiction's to confirm.

Step 6. Save, if asked

Ask whether to write the study to a file and where.

Rules

• Never recall an existing-building threshold or provision. Quote it.
• Run existing-conditions-survey first, or say plainly that the basis is unverified.
• Settle the relationship before any joint detail.
• Work all six systems across the joint, every run.
• Treat the triggers as their own exercise, never as a note under the code analysis.
• Recommend confirming triggers with the authority before the scheme is fixed.
• Never assume existing capacity: structural, electrical, mechanical or drainage.

Anti-patterns

• Designing against record drawings.
• Choosing the relationship by default, which usually produces a matching addition that does not match.
• Drawing the joint as a line on the plan.
• Assuming the existing panel, boiler or drain can take the new work.
• Setting the new floor level and discovering the existing one is not level.
• Finding a trigger at permit.
• Running a finish across the joint between two structures that move differently.
• Treating a designated building's addition as a free design choice.

Resources

None. This skill is one file. Output is written directly at the path you choose.