Decisions 1 to 3: what your site is actually doing

1. Drawing without a site survey. A registered site survey (also called a feature and level survey) costs $1,500 to $3,500 depending on your block. It tells the drafter exactly where the boundaries are, where existing structures sit, and what the ground levels are doing across the site. Without one, the drafter is guessing. We reviewed plans last year drawn from Google Earth imagery where the boundary was 1.2 metres off the real cadastral position. The owner-builder discovered this when the surveyor came to set out the building and the entire footprint sat inside the side setback. Moving the building at that point cost $11,000 in re-engineering and council resubmission. A $2,500 survey would have prevented it.

2. Wrong finished floor level. This is the most expensive single drafting mistake we encounter. The finished floor level (FFL) determines everything below it: the slab design, the fill requirements, the retaining, the drainage, and the accessibility compliance. If the FFL is set too low, you need retaining walls and additional fill. Too high, and you may breach height limits or create accessibility problems with excessive ramp gradients. In flood-affected areas, the FFL must sit above the defined flood level, which requires a flood study from council. The shed to dwelling conversion Australia guide covers this in more detail, because shed conversions on rural properties are where we see this mistake most often. The cost range for fixing it during construction is $15,000 to $60,000 depending on the severity and the site conditions.

3. Setback dimensions wrong. Every lot has setback requirements defined by the local planning scheme. Front, side, and rear setbacks determine where on the block you can build. A drafter who measures from the wrong reference point (the fence line instead of the cadastral boundary, for example) can position the entire building in the wrong place. If the certifier catches it before construction, you lose time and re-documentation fees of $3,000 to $5,000. If it is caught during a frame inspection, you lose part of the building. We have seen a wall demolished and rebuilt 400mm inward because the side setback was measured from the fence, which sat 400mm inside the property boundary. The total cost of that correction was $14,000. 

Decisions 4 to 7: layout choices that compound

4. Poor building orientation. In most Australian climate zones, your living areas should face north. Bedrooms perform best facing south or east. Minimal glazing should go on the west wall. These are not design preferences. They are energy efficiency fundamentals that directly affect your NatHERS rating and, by extension, the insulation specification, glazing type, and mechanical heating or cooling you need to achieve compliance. A badly oriented plan that scrapes through with a 6-star rating may require $10,000 to $15,000 in upgraded glazing and additional insulation to get there. The same plan rotated 90 degrees on the block might achieve 7 stars with standard materials. That rotation costs nothing on a drawing. It costs $5,000 to $15,000 if you try to compensate for it with better-performing materials after the fact.

5. Rooms too small for standard furniture. A bedroom drawn at 2.7 metres by 3.0 metres looks adequate on a floor plan. Then you try to fit a queen bed (1530mm by 2030mm), two bedside tables, and a wardrobe. It does not work. A living room drawn at 3.6 metres by 4.0 metres cannot fit a standard three-seat sofa and a television at a comfortable viewing distance. These are basic dimensional checks that should happen during drafting. When they do not, the owner-builder discovers the problem during framing and asks the trades to move a wall. That wall move costs $3,000 to $8,000 depending on the structural implications. If you are pricing rooms by the square metre, as most 2 bedroom granny flat prices Victoria quotes tend to show, undersized rooms still cost almost the same to build as properly sized ones. You just get less usable space for the same money.

6. Excessive corridor space. Corridors are the most expensive floor area in any building because they serve no function beyond connecting rooms. Every extra metre of corridor is a metre of floor area you paid to frame, clad, insulate, line, and finish, but cannot furnish or use. On a standard build at $2,200 to $2,800 per square metre, a hallway that is 1.5 metres longer than necessary costs $3,300 to $4,200 in wasted construction cost. On a 100-square-metre dwelling, if corridors consume 18% of the floor area instead of 12%, that is 6 square metres of dead space, worth $13,200 to $16,800 in construction cost. Good layout design minimises corridors. Cheap plans do not think about it.

7. Wet areas on opposite sides of the plan. Every wet area (bathroom, ensuite, kitchen, laundry) needs hot water, cold water, and drainage connections. When all the wet areas share a common plumbing wall or sit back-to-back, the plumber runs one set of pipes through one wall cavity. When the bathroom is at the front of the house and the laundry is at the back, the plumber runs two separate sets of pipes across the full length of the building. That adds $6,000 to $18,000 in plumbing labour and materials, depending on the distance and whether you are building on a slab or a suspended floor. It also increases the number of potential leak points over the life of the building.

Decisions 8 to 10: when the structure fights the plan

8. Non-standard window and door sizes. Australian window manufacturers produce standard sizes in modular increments. A 1200mm by 1200mm awning window is a catalogue item that costs $280 to $450 depending on frame material and glazing type. A 1350mm by 1150mm window is a custom order that costs $600 to $900 and takes two to four weeks longer to arrive. If your drafter specifies non-standard sizes across twenty openings, the custom fabrication premium adds $4,000 to $12,000 to your window package alone. The lead time can also push back your lockup stage by weeks, which in turn delays your plasterer, your painter, and your kitchen installer. Standard sizes exist for a reason. Use them unless the design genuinely requires something different.

9. Layout ignores the structural grid. Structural engineers design beams, columns, and bracing on a grid. When the architectural layout aligns with that grid (load-bearing walls on grid lines, openings positioned between grid points), the structural design is efficient. When the layout places openings, cantilevers, and load-bearing walls in random positions, the engineer has to design around the architecture with heavier solutions. That means bigger beams, more columns, deeper footings, and more steel. The cost difference between a structurally aligned layout and a random one is $5,000 to $15,000 in steel and concrete alone. This is a design-stage decision that costs nothing to get right and thousands to get wrong. The same principle applies at any scale. 

10. Missing cross-sections and construction details. Plans that only include floor plans and elevations are not complete. Cross-sections show the vertical relationships: floor-to-ceiling heights, roof pitch, footing depth, slab edge detail, and how the building sits on the ground. Construction details show how materials connect at junctions: wall to floor, window head and sill, flashing, and waterproofing layers. Without these, your trades are guessing. A framer who guesses wrong about the wall-to-roof junction costs you $3,000 to $10,000 in rework. A tiler who waterproofs to the wrong height costs $5,000 to $15,000 in strip-out and redo. These drawings take the drafter a few hours to produce. Leaving them out costs you weeks on site.

Decisions 11 to 13: services nobody thought about

11. No engineer coordination at draft stage. If your drafter completes the full set of plans and then sends them to the structural engineer after the fact, the engineer designs the structure to suit the architecture as best they can. Often it does not go smoothly. The engineer may require a column in the middle of an open-plan living room, a deeper beam that drops below the ceiling line, or a footing system that conflicts with the drainage design. If this happens after council approval, you are looking at amended plans, a new DA or CDC submission, and construction delays. The cost runs $10,000 to $40,000 depending on the extent of the changes. The fix is coordination. The drafter and engineer should be talking during the design phase, not after it. This is especially critical for projects where existing structure constrains the engineering options.

12. Stormwater and drainage missing from the site plan. Council requires a stormwater management plan for any new dwelling or significant alteration. If the drafter does not include stormwater design in the documentation, you will be asked for it during the approval process, or worse, during a construction inspection. Retrospective stormwater design costs $2,500 to $5,000 for the engineering and documentation alone. If the drainage path conflicts with the approved slab or footing design, the construction cost to resolve it runs $8,000 to $25,000. On sloping sites, where stormwater management is most critical, the drainage design needs to be resolved before the slab detail is finalised. Not after. The 10 shed conversion mistakes that cost $20K+ to fix later covers several drainage-related errors that overlap with this one.

13. Ceiling heights below NCC minimums. The National Construction Code (NCC) sets minimum ceiling heights for habitable rooms at 2,400mm and for non-habitable rooms (bathrooms, laundries) at 2,100mm. These are not suggestions. If your ceiling heights fall below these minimums, the certifier will reject the inspection and you will need to raise or rebuild the roof framing. On a shed conversion where the existing roof height constrains the internal ceiling, this mistake is catastrophic. Raising a roofline after framing costs $15,000 to $50,000 in structural steel, roofing materials, and labour. In some cases, it is cheaper to strip the roof entirely and start again. The minimum heights should be confirmed at the drafting stage, with a surveyor’s RL (reduced level) to prove the existing structure has the clearance. If it does not, you know before you spend anything on construction, not after.

Decisions 14 to 15: the documentation gap

14. No electrical layouts on plans. Plans that show room layouts but not powerpoint positions, switch locations, or light fittings leave every services decision to the trades on site. Electricians are skilled, but they are not designers. Without a layout, your electrician will place switches where it is easiest to run cable, not where it makes sense for the room. The result is outlets behind furniture and switches on the wrong side of the door. Each individual fix costs $300 to $800. Across a full dwelling with 40 to 60 electrical points, the cumulative rework cost is $3,000 to $8,000.

15. Incomplete documentation for the certifier. A building certifier (private or council) needs a specific set of documents to issue a construction certificate or building permit. At a minimum, this includes architectural plans, structural engineering, energy efficiency assessment, site classification report, stormwater design, and a BAL (Bushfire Attack Level) assessment if applicable. If your documentation set is missing any of these, the certifier will reject the application and you wait. Each missing report costs $1,500 to $4,000 to commission and adds two to six weeks to your approval timeline. If you have already engaged trades and ordered materials based on an assumed start date, the delay alone can cost $5,000 to $20,000 in holding costs, rescheduling fees, and material price increases.