Planning an extension: foundations, slabs & costs—a 2025 guide before breaking ground

An extension, garage conversion, or granny flat represents one of the most significant financial and structural investments a homeowner will make. While the dream is of more space, the 2025 reality is a complex journey of legal, technical, and financial hurdles. This guide is not just about what to build, but how to build it correctly in South Africa.

The success of any extension project hinges on three pillars:

• Legal & Administrative Compliance: Navigating a municipal approval system that is in a widely reported state of "crisis".
• Technical Excellence: Ensuring the new foundations, slabs, and structure are 100% compliant with the National Building Regulations (SANS 10400) and NHBRC standards.
• Financial Control: Moving from a vague "cost per square metre" to a robust budget that accounts for professional fees, hidden costs, and mandatory contingencies.

This guide provides the expert-level knowledge necessary for a homeowner to act as an informed project owner. It walks through every stage, from the first feasibility check to the final compliance certificate, equipping you to ask the right questions, hire the right team, and transform a vision into a safe, legal, and valuable asset.

The feasibility check: what can be built, and where

Before engaging an architect, a homeowner must first conduct a preliminary investigation. A property's legal documents—not a builder's opinion—dictate what is possible.

Your property's 'rulebook': title deed vs. zoning

A property's development potential is governed by several legal layers.

• Zoning Certificate: This document from the local municipality defines the property's land use and governs critical parameters like Coverage (the percentage of the plot covered by buildings), Floor Area Ratio (FAR), and Building Lines (the 'no-build' zones).
• Title Deed: This document may contain further restrictive conditions. A common example is a covenant limiting the property to a single dwelling.

The most restrictive rule always applies. For example, a property's zoning might permit a 120m² second dwelling. However, if the Title Deed contains a restrictive covenant limiting the property to one dwelling, the Title Deed overrides the zoning. The homeowner must first apply for the removal of this restriction—a costly and lengthy process—before an extension can be considered.

For properties within a complex or estate, the Body Corporate or Homeowners' Association rules are a third, powerful layer of law. New directives from the Community Schemes Ombud Service (CSOS) grant these bodies significant power and enforce strict compliance.

Granny flats vs. second dwellings: a critical definition

Municipalities have specific definitions for these structures. A "granny flat" or "accessory dwelling unit" is often limited in size, typically to 60m², although some metros may allow up to 120m². Key requirements almost always include a separate, independent entrance and, crucially, separate utility connections or sub-meters for water and electricity. This has direct cost implications that must be budgeted for.

The 60-year rule: is the home a heritage site?

Under the National Heritage Resources Act, any structure older than 60 years is protected, regardless of its apparent historical significance. This means any alteration requires a permit from the South African Heritage Resources Agency (SAHRA) or the relevant provincial body.

A 1963 suburban home may not look like a monument, but if it is over 60 years old, it is protected. The local municipality cannot legally approve building plans for such a property without a corresponding approval from the heritage authority.

Site & services audit

An architect must draw a Site Development Plan (SDP) showing all building lines. If a design must cross this line, a separate "Building Line Relaxation" application must be submitted, which can add months to the timeline.

The location of existing sewer, water, and electrical lines is also critical. An extension cannot be built over a municipal sewer line. A simple garage conversion will require digging up the new floor to reroute plumbing or upgrading the electrical distribution board to handle the new load.

Assembling the professional team: who to hire and why

The most common mistake on a residential project is letting a builder "handle the plans." This approach often leads to non-compliant designs, failed submissions, and a complete lack of accountability. A professional project requires a professional, registered team.

The core team: architect vs. structural engineer

• The Architect (SACAP-Registered): The architect is the lead designer and primary agent for the homeowner, registered with the South African Council for the Architectural Profession (SACAP). Their role is to design a functional space, produce council-ready drawings compliant with SANS 10400, and navigate the complex municipal bureaucracy.
• The Structural Engineer (ECSA-Registered): The engineer is the project's structural guardian, registered with the Engineering Council of South Africa (ECSA). Their involvement is non-negotiable. They assess the existing structure, calculate loads for the new extension, and provide specific structural drawings and compliance forms.

The architect and engineer work in tandem. The architect designs the form (e.g., "an open-plan living area"), and the engineer determines the function (e.g., "the structure requires a 300mm steel beam and a 1.5m x 1.5m concrete pad footing").

The financial guardian: the quantity surveyor (QS)

A Quantity Surveyor (QS) is a construction cost consultant. While often skipped on smaller projects, their involvement is invaluable for a large extension. A QS protects the homeowner's finances by creating a detailed Bill of Quantities (BoQ) for accurate pricing, identifying potential cost overruns, and managing monthly payments to the contractor.

Alternative model: architect vs. design-build contractor

• Traditional (Architect-led): The homeowner hires an architect who designs the project. These plans are then put out to tender with several builders. The architect acts as the homeowner's advocate. This model provides competitive pricing but can be slower.
• Design-Build (Contractor-led): The homeowner hires one firm that has in-house designers and manages the entire build. This offers a single point of contact and often a fixed price. However, the homeowner loses the architect as their independent advocate.

2025 professional fee guidelines

Professional fees are not fixed but are guided by their respective councils. Homeowners should budget approximately 8–15% of the total build cost for these "soft costs".

The municipal gauntlet: navigating plan approvals in 2025

Not all work requires a full plan submission. The National Building Regulations (NBR) allow for "minor building work," such as small tool sheds or pergolas. However, permission is still required. A common trap is assuming a garage conversion is minor. Changing a garage door for a window and wall is a fundamental change of use and requires a full plan submission.

The submission checklist

The architect will compile the final submission pack. A primary cause of rejection is an incomplete submission. The homeowner will need to provide several key source documents.

The 2025 approval crisis: a special report

The single biggest risk to an extension project in 2025 is the municipal approval timeline. The official-but-rarely-met timeline is 30–60 days. The 2025 reality is that systemic, severe delays are the new normal. Homeowners and professionals across the country are reporting 6- to 12-month (or longer) waits for plan approval.

The South African Council for the Architectural Profession (SACAP) declared a "Planning Application Crisis," citing "excessive delays," "maladministration," and "stalled projects worth billions" as evidence of a systemic failure.

For a homeowner, this risk must be actively managed. The project timeline must assume a 6- to 12-month approval phase. The budget must include a contingency for builder price escalations during this unavoidable delay. In 2025, an architect's most valuable skill is not just their design, but their experience in navigating these digital submission portals and their professional relationships at the council.

The build part 1: foundations, slabs, and damp

The foundation is the single most important part of the build. It is engineered, not guessed. An incorrect foundation will lead to structural failure, and insurance claims will be rejected if the structure is found to be non-compliant.

Understanding the site: NHBRC classifications

The structural engineer will classify the site according to the National Home Builders Registration Council (NHBRC) standards, based on soil type. The foundation type is not a choice; it is a prescription dictated by these soil conditions.

Foundation types explained

• Strip Footings: The "traditional" foundation, consisting of concrete strips under load-bearing walls. This method is only suitable for stable soil.
• Slab-on-Ground (Raft Foundations): A single, large concrete slab that "floats" on the ground. This is the required solution for expansive clay or poor soils, as it distributes the building's load over a wider area.

SANS 10400 damp-proofing standards: a non-negotiable

Moisture will penetrate a concrete slab unless a legal-standard damp-proof course (DPC) is used. The SANS 10400 specification is precise: a 250µm (micron) polyolefin membrane must be laid under the entire slab on a clean bed of sand. The membrane must be turned up the foundation wall and bonded to the horizontal DPC in the brickwork to create a continuous waterproof "box". A common, non-compliant shortcut is using thin black plastic, which will lead to rising damp and leave the homeowner with no legal recourse.

The build part 2: structure, tie-ins, and crack prevention

Going up? assessing loads for a second storey

A second floor cannot simply be added to an existing structure. An ECSA-registered engineer must be hired to assess if the original foundations can carry the additional load. This often involves digging "test pits" to physically inspect the foundation.

A common trap is the plan to build over an existing garage. A homeowner may see a "flat concrete roof" and assume it can serve as a floor. In almost all cases, it cannot. The garage's foundations were likely designed only for a light roof. The solution (e.g., underpinning or adding new columns) can be prohibitively expensive.

Joining new to old: the technical guide to crack prevention

The most common defect in an extension is a vertical crack where the new building joins the old. This is caused by "differential settlement"—the new structure settles into the ground at a different rate than the old one.

Simply "butting" new concrete against the old is a guaranteed failure. The new foundation and slab must be physically tied to the existing structure using a "drill and dowel" method specified by an engineer.

The builder must drill holes into the existing foundation, inject a high-strength structural epoxy, and insert new rebar "dowels". The rebar for the new extension is then tied to these dowels, creating a single structure that settles as one unit. New walls must also be built according to SANS 10400-K to prevent cracking, including the correct use of "brickforce" and "control joints".

The build part 3: services, safety, and final compliance

Rerouting services

Services are a major cost and must be planned early. A new bathroom on the far side of the house may require breaking up existing floors to run new sewer lines. A garage conversion will require new hot and cold water supplies, plus a new waste drain. Electrically, the single circuit for a garage light must be upgraded to handle multiple plugs and lights, requiring a new circuit from the main distribution board.

Electrical certificate of compliance (CoC)

An Electrical CoC is a legal requirement under the Occupational Health and Safety (OHS) Act. Any alteration to an electrical system requires a new CoC. A registered electrician inspects the new work for compliance with SANS 10142-1, checking breakers, earthing, and isolators. A common mistake is skipping the CoC. Years later, when the owner tries to sell, the buyer's bank demands a CoC, and the owner is now responsible for fixing any non-compliant work.

Plumbing certificate of compliance (CoC)

A plumbing CoC, issued by a PIRB-registered plumber, is also mandatory for specific work. It is legally required when selling a home and after "major plumbing renovations" or the installation of a new geyser. The plumber certifies that the hot water cylinder is compliant, pipes are secure, there are no leaks, and stormwater is not being discharged into the sewerage system.

2025 cost analysis: budgeting an extension

The most common metric for building costs is Rand (ZAR) per square metre (m²). This rate, however, is not a quote. It is a broad estimate based on the specified level of finishes.

Cost bands for home extensions (2025)

The hidden costs & the contingency fund

A builder's quote is not the final price. The project budget must include "soft costs" and a fund for unknowns.

• Professional Fees: 8–15% of the build cost (for architect, engineer, etc.).
• Municipal Fees: Plan approval fees, plus connection fees for new water or electrical meters.
• Utility Rerouting: The cost to move an unexpected sewer pipe or electrical main.
• Contingency Fund (15–20%): This is not optional. It is a mandatory fund for unforeseen issues, such as hitting hard rock during excavation or uncovering non-compliant wiring.

The contingency fund is not optional. It is a mandatory fund for unforeseen issues, such as hitting hard rock during excavation or uncovering non-compliant, dangerous wiring in the old structure.

Sample budget for a 30m² standard extension (Gauteng)

This example illustrates how a base "build cost" relates to the total project budget.

Project timeline: from concept to completion

Optimistic timelines are a common source of conflict. The 2025 reality requires budgeting for significant administrative time before any construction begins.

• Phase 1: Planning & Design (1-4 Months): This involves appointing the professional team, conducting surveys, and finalizing all drawings for submission.
• Phase 2: Municipal Approval (3 - 12+ Months): This is the "dead time" where the project is in the municipal system. The 2025 reality is a 6- to 12-month wait. No builder should be scheduled until approved plans are in hand.
• Phase 3: Construction (3-10 Months): The actual build time depends on size and complexity. A small extension may take 3–4 months, while a large, double-storey extension can take 6–10 months.

The next step: from plan to project

This guide has outlined the significant risks of undertaking an extension in 2025. These can be summarized as legal, administrative, technical, compliance, and financial risks. These risks are not manageable with an unvetted or informal builder. The only way to protect such a large investment is to work with a team of qualified, registered, and insured professionals.

This guide provides the what and the why. The next logical step is to find the who. A platform for sourcing quotes serves as a critical tool for this, allowing homeowners to source multiple, obligation-free quotes from a network of:

• SACAP-Registered Architects who can design the space and navigate the municipal gauntlet.
• ECSA-Registered Engineers who will ensure the structure is safe and compliant.
• NHBRC-Registered Builders who have been vetted and have a track record of quality.
• PIRB & ECA-Registered Plumbers and Electricians who can issue the final, legal compliance certificates.

A project can be started with confidence by obtaining and comparing quotes from vetted professionals.