Modifying House Plans: What's Possible and What to Avoid

When embarking on the journey of building your dream home, finding a house plan that meets all your needs can be challenging. While many homeowners find plans that come close to their vision, most require some modifications to truly become the perfect fit. Understanding what modifications are practical, cost-effective, and structurally sound can save you time, money, and frustration during the building process.

Understanding House Plan Modifications

House plan modifications range from simple cosmetic changes to complex structural alterations. Before diving into specific modifications, it's important to understand that house plans are intricate systems where each element serves a purpose and relates to other components. When we modify one aspect, we often create a ripple effect that impacts other areas of the design.

What Modifications Are Typically Possible

Minor Modifications

Minor modifications generally don't affect the structural integrity or overall footprint of the house. These changes are usually straightforward and cost-effective:

1. Interior wall adjustments: Non-load-bearing walls can often be moved, removed, or added to reconfigure spaces within the existing footprint.
2. Window and door relocations: Changing the size, style, or location of windows and doors is typically feasible, provided they don't affect load-bearing walls.
3. Ceiling height changes: Raising or lowering ceiling heights in specific rooms can create different spatial experiences, though this may affect HVAC considerations.
4. Finish selections: Changing materials for flooring, countertops, cabinetry, and other finishes is always an option that doesn't affect the structural design.
5. Built-in features: Adding or removing built-ins like bookshelves, window seats, or storage nooks usually doesn't impact structural elements.

Moderate Modifications

These modifications may have structural implications but are still generally feasible:

1. Room extensions: Pushing out a wall to enlarge a room often requires structural analysis but is usually possible with proper engineering.
2. Foundation changes: Switching from a slab to a basement or crawl space (or vice versa) requires significant redesign but is achievable with professional assistance.
3. Roof modifications: Changing roof pitches or styles may affect the entire structural system but can be accommodated with proper engineering.
4. Adding or expanding garages: Modifying garage spaces often involves foundation and structural changes but is a common modification.
5. Enlarging or combining rooms: Creating open-concept spaces by removing walls requires careful structural analysis to ensure proper support.

Major Modifications

Major modifications essentially transform the original plan and often require extensive redesign:

1. Adding a story: Converting a single-story plan to two stories involves comprehensive structural redesign.
2. Significant footprint changes: Dramatically altering the shape or size of the house requires extensive redesign of foundations, roof systems, and structural supports.
3. Reversing or mirroring plans: While seemingly simple, this affects everything from plumbing and electrical systems to window placements and roof designs.
4. Changing architectural style: Transforming a contemporary design to traditional (or vice versa) often involves fundamental changes to rooflines, window patterns, and structural systems.

What to Avoid When Modifying House Plans

Code Compliance Issues

Modifications should never result in code violations:

1. Reducing egress options that are required for safety
2. Compromising fire separations between spaces
3. Creating rooms without required ventilation
4. Reducing stair widths or increasing riser heights beyond code limits
5. Violating setback requirements with additions or extensions

Functional Problems

Avoid modifications that create functional challenges:

1. Disrupting traffic flow through awkward room arrangements
2. Creating dead-end spaces that serve no purpose
3. Reducing closet or storage space without alternatives
4. Placing bathrooms in inappropriate locations (such as directly off living or dining areas)
5. Creating kitchens with inefficient work triangles

Energy Efficiency Compromises

Don't make changes that significantly reduce energy efficiency:

1. Excessive window area on harsh exposures without proper glazing specifications
2. Reducing insulation opportunities in exterior walls
3. Creating thermal bridges through poor detailing
4. Compromising HVAC system effectiveness through inappropriate room configurations

The Modification Process with The House Plan Company

According to The House Plan Company, modifying house plans involves a systematic approach to ensure success:

Step 1: Pick a Plan

Browse their collection of expertly designed homes, garage and multi-family plans. Once you find that plan that catches your imagination, prepare your desired list of changes.

Step 2: Contact Them

Whether you prefer to communicate via email or phone, their team of client support specialists are ready to help you kick off the modification quote process.

Step 3: Quote

Modifications are sent to an HPC designer to review and prepare a FREE modification quote. Once you have worked out the customizations and are ready to move forward HPC’s team will help you through the order process.

Step 4: Design

Your modification estimate is approved, your plan package purchase has been made you are off and working with an HPC designer who will help you create your dream plan.

Cost Considerations: Professional Design Services

Understanding the costs associated with professional design services is crucial when budgeting for house plan modifications. According to data from the American Institute of Building Design (AIBD), design service costs vary based on project complexity, scope, and regional factors.

AIBD Cost Guidelines for House Plan Modifications

The AIBD provides the following cost estimates for different levels of modification services:

1. Minor Modifications ($500-$2,500)
• Simple non-structural changes like moving interior non-load-bearing walls
• Window or door relocations that don't affect structural elements
• Minor kitchen or bathroom reconfigurations within existing spaces
• AIBD members typically charge $75-$150 per hour for these services
2. Moderate Modifications ($2,500-$7,500)
• Room additions under 400 square feet
• Converting garage space to living areas
• Significant interior reconfiguration with some structural implications
• Foundation adjustments for specific areas
• AIBD members typically charge $100-$175 per hour for these services
3. Major Modifications ($7,500-$15,000+)
• Adding a full second story
• Extensive structural changes affecting multiple systems
• Changing the architectural style of the home
• Significant footprint alterations
• Complex roof system modifications
• AIBD members typically charge $125-$200+ per hour for these services

Fee Structure Options

According to the AIBD, design professionals typically offer several payment structures:

1. Hourly Rates: The most common approach for modifications, with rates ranging from $75-$200+ per hour depending on the professional's experience, credentials, and regional market conditions.
2. Fixed Fee: Often calculated as a percentage of construction costs:
• Minor modifications: 1.5-3% of construction costs
• Moderate modifications: 3-6% of construction costs
• Major modifications: 6-12% of construction costs
3. Square Footage Basis: Some AIBD members charge by the square footage affected:
• $1.25-$3.50 per square foot for minor modifications
• $3.50-$7.00 per square foot for moderate modifications
• $7.00-$12.00+ per square foot for major modifications or custom designs

Additional Cost Factors

The AIBD notes several factors that can influence modification costs:

1. Geographic Location: Design services in metropolitan areas typically cost 20-40% more than in rural areas.
2. Project Complexity: Unique or challenging modifications require more design time and expertise.
3. Timeline Requirements: Expedited services often incur premium charges of 25-50%.
4. Number of Revisions: Most design contracts include 2-3 revision cycles, with additional revisions billed separately.
5. Permit Documentation: Preparation of comprehensive documentation for building permits may add $1,000-$3,000 to the overall design costs.

Construction Cost Impacts

Beyond design fees, modifications affect actual construction costs:

1. Minor modifications typically add 5-10% to construction costs
2. Moderate modifications may add 10-20% to construction costs
3. Major modifications can increase construction costs by 20-50% or more

The AIBD emphasizes that investments in professional design services typically represent only 3-8% of total project costs but can prevent costly construction errors and optimize the functionality and value of your home.

Modifying house plans allows you to personalize your dream home, but approaching modifications with knowledge and professional guidance is critical. By understanding what's possible, what to avoid, and working with experienced professionals like those at The House Plan Company, you can create a customized home that meets your needs without compromising structural integrity, functionality, or budget.

The AIBD cost guidelines provide valuable insights into budgeting for professional design services. While these services represent an additional investment, they ensure that modifications are properly designed, documented, and engineered—potentially saving thousands in construction costs and preventing future problems.

Remember that the most successful house plan modifications balance your vision with practical realities, resulting in a home that is not only beautiful but also structurally sound, functional, and built to last. Before making any modifications, consult with professionals who can help navigate the complex interplay of design, structure, codes, and constructability.

By following these guidelines and working with experienced professionals, you can transform an existing house plan into your perfect home while avoiding costly mistakes and disappointments along the way.

References:

[1] American Institute of Building Design. (2023). "Budgeting for Design Services: A Consumer Guide." AIBD Consumer Resource Series.

[2] American Institute of Building Design. (2024). "Residential Design Service Contract Guidelines." AIBD Professional Practice Publications.

[3] American Institute of Building Design. (Spring 2024). "Cost Factors in Residential Modifications." Design Lines Quarterly, Vol. 42, Issue 2.

[4] American Institute of Building Design. (2023). "Understanding Residential Design Costs: From Plans to Construction." AIBD Consumer Education Series.