How Drawing Review Caught 5 Conflicts on a Detroit Mixed-Use Rehab

A Historic Building, a New Addition, and Ground Floor Retail in Detroit

A developer in Detroit was preparing to build a mixed-use project: apartments in a rehabilitated historic building and a new addition, with retail on the ground floor and a corner space set aside for a future restaurant. The financing counted on federal and Michigan historic tax credits for the historic portion.

Projects like this carry more parties than a typical apartment building. The city reviews stormwater, the National Park Service reviews the historic work, DTE designs the electrical service, and retail tenants bring their own lease requirements. Each one shows up in a different part of the drawing set.

The developer wanted the gaps between those pieces found before the foundation was poured and before any leases were signed.

Helonic Found Louvers That Put the Historic Tax Credits at Risk

The developer uploaded the PDF set, the specifications, and the retail lease exhibits. Helonic compared the civil, architectural, mechanical, plumbing, and electrical drawings with each other and with the documents the project had already committed to.

The first finding was a restaurant with no grease duct shaft. The ground floor plan on A-101 labeled a corner space as a future restaurant tenant, and the mechanical notes said kitchen exhaust was by the tenant. The residential floor plans stacked apartments directly over that corner with no shaft.

A commercial kitchen hood needs a grease duct that runs continuously to the roof inside a fire-rated shaft, with clearance to combustible construction and access panels at changes in direction. Without a shaft carved out of the floors above, the restaurant cannot be built. The shaft also has to clear structure and land on a rooftop fan location, and the roof plan was using that area for residential equipment.

Found after the apartments are framed, the fix takes space out of a stack of units, which can mean losing rentable area or a whole unit line, plus reframing floors and building a rated shaft through finished construction. Found after a lease is signed, it can lose the restaurant. The developer priced it at $100,000 to $350,000 in the field, plus lost rent from any unit removed, with 3 to 8 weeks of delay. Fixed before construction, it was $3,000 to $10,000.

The second was storm drains that bypassed the detention system. Detroit's public sewer is largely a combined system, and the city's stormwater rules for new development require on-site management, typically underground detention with a restricted release rate. Civil sheet C-301 showed an underground detention system under the parking area, with its inlet invert at a set elevation.

Plumbing sheet P-001 brought the building storm main out through the foundation wall at its own invert and connected it straight to the combined sewer lateral at the street. That bypassed the detention system, which violates the stormwater approval. Even rerouted, the storm main left the building lower than the detention inlet, so it could not drain into it by gravity. Plumbing and civil had each drawn their own side of the 5-foot line outside the building.

Fixing inverts after the foundation and underground utilities are in means digging the site back up, possibly adding a storm pump station, or redesigning the detention system, and it can hold up the city's final stormwater approval and the certificate of occupancy. The developer priced it at $50,000 to $250,000 with 2 to 6 weeks of delay. Fixed before construction, it was $2,000 to $8,000.

The third was new louvers in the historic facade. The architectural elevations on A-201 and the historic tax credit application both committed to keeping and restoring the original masonry facade and windows. Mechanical sheet M-201 put a through-wall PTAC or ventilation louver in every apartment, cut into that facade, and plumbing ran dryer vents through the same wall.

The federal historic tax credit requires the work to meet the Secretary of the Interior's Standards for Rehabilitation and to be approved by the National Park Service. New openings in a primary historic facade are one of the most common reasons a project is denied or sent back for redesign. The mechanical approach that worked for the new addition had been copied into the historic building.

The federal credit alone is 20% of qualified rehabilitation costs, and Michigan has its own state historic credit, so the credits were part of how the project was financed. Switching to a system that vents through the roof or secondary elevations after rough-in means redesigning and rerunning ductwork in every historic unit. The developer priced it at $150,000 to $500,000 in rework, plus the risk of losing credits worth millions, with 4 to 12 weeks of delay plus National Park Service review time. Fixed before construction, the system redesign was $5,000 to $15,000.

The fourth was the electrical service and the transformer. The spec section 26 27 13 and the retail lease exhibits required separate utility meters for each retail tenant and each apartment. The electrical one-line on E-601 showed a single service and a single meter bank sized for the apartments only, with no metered service for retail and no capacity reserved for a restaurant kitchen.

DTE, the utility for most of the Detroit area, has its own requirements for transformer location, clearances, truck access, and metering equipment, and the drawings did not account for them. The civil and landscape drawings put a plaza with planters and seating where the pad-mounted transformer would need to go.

Utility design and equipment lead times are some of the longest on any job, often months for transformers and switchgear. A late change to the service can be the one item that delays permanent power, and without permanent power there is no certificate of occupancy and no tenant move-in. The developer priced it at $75,000 to $300,000 for switchgear and meter changes, transformer relocation, and site rework, with 6 to 20 weeks of delay. Fixed before construction, it was $2,000 to $8,000.

The fifth was heat pumps sized for a mild day. The spec section 23 81 26 required cold-climate heat pumps or VRF that deliver full heating capacity at the local winter design temperature, which in Detroit is in the low single digits Fahrenheit. The mechanical schedule on M-601 listed standard heat pumps, with heating capacity shown at the standard 47°F rating condition and no supplemental electric heat.

Standard heat pumps lose a large share of their heating capacity as outdoor temperatures drop toward 0°F, so units sized at 47°F leave apartments underheated on the coldest days. The fix is either cold-climate equipment, often with a larger electrical service, or electric heat strips, which change every unit's circuit and the building's total load. Either way it runs into the electrical service problem above.

Installed as drawn, the problem shows up the first January as tenant complaints and warranty claims. The developer priced it at $100,000 to $400,000 to swap equipment or add heat strips and circuits in the field, with a similar cost after occupancy plus complaints and vacancies. Fixed before construction, it was $1,000 to $5,000.

See mixed-use construction drawing review, Michigan plan review requirements, adaptive reuse drawing challenges, stormwater management drawing errors, mixed-use vertical stacking coordination, vertical stacking analysis, and the mixed-use development review.

$475,000 to $1.8 Million in Field Rework Fixed for $13,000 to $46,000

Helonic flagged all five before construction, while each fix was still a drawing revision.

Fixed at that stage, the five came to $13,000 to $46,000. Found in the field, the developer priced the same five at $475,000 to $1.8 million, before lost rent, a lost restaurant tenant, or lost historic tax credits.

The stormwater finding was caught before the foundation was poured, when the inverts were still lines on two sheets.

The storm main and the detention system were each drawn correctly on their own sheets and wrong together. Across 1,000+ drawing packages Helonic has reviewed, about 8 in 10 had at least one coordination conflict between disciplines. See what Helonic found across 1,000+ drawing reviews.

Grease Ducts, Detroit Stormwater Rules, and the Secretary of the Interior's Standards

Michigan adopts statewide codes based on the International Codes, including the Michigan Building Code, the Michigan Mechanical Code, and the Michigan Plumbing Code. Detroit enforces them through its own building department, and the Detroit Water and Sewerage Department reviews stormwater for new development.

Commercial kitchen grease ducts fall under the mechanical code and NFPA 96. The duct runs continuously to the outside, is enclosed in a fire-rated shaft where it passes through floors, keeps clearance to combustible construction unless a listed wrap reduces it, and has cleanout access at changes in direction.

The federal historic tax credit pays 20% of qualified rehabilitation costs when the work meets the Secretary of the Interior's Standards for Rehabilitation. The state historic preservation office reviews the application first, and the National Park Service makes the final decision. The standards favor keeping historic features on primary elevations, which is why new wall openings there draw scrutiny.

Electrical services follow the National Electrical Code as adopted in Michigan, but the service equipment, metering, and transformer location are set by the utility's own standards. DTE's requirements for clearance and truck access decide where a pad-mounted transformer can go.

Heat pumps are rated at 47°F, with a second heating rating at 17°F. Equipment for a Detroit apartment has to be selected at the local winter design temperature, which is in the low single digits Fahrenheit, not at the rating point.

“Our plumbing drawings ran the storm main straight to the street, past the detention system the city required, at an elevation that couldn't reach it anyway. Helonic found it before the foundation was poured. In Detroit, that's the difference between a quick fix and digging up the site.”

Detroit Mixed-Use Historic Rehab Drawing Review: Questions and Answers