Most flip disasters don’t happen during demo. Or when the contractor ghosts. Or even when the market dips mid-project.

They happen before the first hammer hits the drywall.

Because bad underwriting = bad outcomes, no matter how good your team is, how pretty the house looks, or how hot the zip code is.

In this guide, we’ll break down the 3 most common underwriting mistakes flippers make, why they happen, how to spot them early, and most importantly, how to avoid them entirely.

These are the killers that lurk in the spreadsheet, and if you know how to catch them early, you’ll save yourself time, margin, and misery.

Mistake #1: Using Fantasy ARVs (Instead of Market-Backed Comps)

The Problem

Most new or overeager investors overestimate After Repair Value (ARV).

Why?

• They use the highest comp instead of the average

• They compare to renovated homes that are larger, newer, or better-located

• They assume the market will rise during the project

• They want the deal to work, so they bend the numbers

The result: your whole underwriting stack is built on fiction.

If your ARV is wrong, every downstream number gets distorted:

• Rehab % based on ARV? Too low.

• Max allowable offer? Too high.

• Holding cost estimates? Off by weeks or months.

• Profit projection? False confidence.

What It Looks Like in the Wild

You underwrite a flip:

• Purchase: $160K

• Rehab: $55K

• Projected ARV: $315K

• Profit: ~$40K after costs

But here’s what you didn’t see:

• The $315K comp was 200 sq ft larger, a corner lot, and better finishes

• Real ARV is more like $285K

• Now your profit drops to ~$15K, or evaporates completely if rehab runs over

That’s how you lose on Day 1.

How to Avoid It

Reverse-engineer comps based on realism, not optimism.

• Pull 3–5 recent, sold comps within 0.5 mi

• Match bedroom count, square footage, lot size, and finish level

• Adjust down for anything your flip won’t have

• Be conservative on timeline: if the market’s slowing, the price for that

Bonus Tip: If your flip will be the nicest house on the block, your ARV isn’t real; it’s retail fantasy.

Mistake #2: Underestimating Rehab Scope (and Not Building Buffers)

The Problem

Flippers, especially spreadsheet warriors, love a tight budget.

The problem? Reality hates tight budgets.

When you underestimate:

• Labor costs

• Material surges

• Permit timelines

• Surprise damage behind walls

• Change orders

…your entire margin gets wiped out.

A $50K rehab that turns into $68K is not rare; it’s the norm for poor underwriting.

And if your underwriting didn’t include a buffer? That money comes out of your profit.

How It Happens

This usually stems from:

• Trusting wholesaler or seller “repair estimates”

• Using $/sq ft averages without a scope walkthrough

• Not factoring in holding costs tied to longer rehab timelines

• Skipping permit or inspection assumptions

• Ignoring seasonal contractor availability or market inflation

What It Looks Like

Your deal:

• Rehab budget: $40K

• Timeline: 8 weeks

• Net profit: $35K

Reality:

• New subfloor needed = +$6K

• Electrical not to code = +$4K

• 3-week permit delay = +$2.5K holding cost

• Material inflation mid-project = +$3K

• Final punch list = +$1.5K

Actual rehab: $57K
Profit left: $18K… maybe

That’s a 50% drop in margin, because you tried to “make the deal work.”

How to Avoid It

Always:

• Get a second rehab bid (even if rough)

• Build a 15–20% contingency into your underwriting

• Assume longer timelines in winter, permit-heavy markets, or older homes

• Use checklists for scope to avoid blind spots

• Ask: “What’s the most expensive thing this house could be hiding?”

Rule of Thumb: If you haven’t physically walked the property with someone qualified, you haven’t underwritten the rehab, you’ve guessed it.

Mistake #3: Misjudging Timeline (and the True Cost of Time)

The Problem

Most flippers treat time as an afterthought in underwriting.

They focus on purchase price, rehab, resale comps…

But forget that time = money, literally.

Every month longer your project takes:

• Your lender charges another interest payment

• Your utilities, taxes, and insurance continue

• Your opportunity cost compounds

• Your profit shrinks, even if everything else goes “as planned”

And if the market shifts mid-project? You’re selling into a weaker ARV.

This is where “good” deals go bad silently.

Why It Happens

Timeline issues stem from:

• Unrealistic rehab durations

• Unaccounted-for permit delays

• Overestimating contractor availability

• Seasonal slowdown (weather or buyer demand)

• Not including DOM + closing time in exit model

• Underwriting as if the project is starting tomorrow (when it’s really 4–6 weeks out)

Example

Let’s say your hard money loan is 12% interest, interest-only, and you’re borrowing $240K.

That’s $2,400/month in interest alone.

You underwrite:

• 3-month rehab

• 1-month listing

• 30-day close

• 5 months total

• $12,000 in interest

But delays add 2 months, and DOM extends another 3 weeks. Now it’s:

• 7 months total

• $16,800 in interest

• Plus extra utilities, insurance, holding costs = +$2,500

• $7,300 loss in pure time-related expenses

That was most of your margin.

How to Avoid It

In underwriting:

• Build in padded timelines, not best-case

• Account for local DOM trends

• Use recent flips in the neighborhood to benchmark rehab duration

• Always include the listing-to-close time in the holding cost

• Avoid deals with tight margins + long timelines unless you’re an expert

Pro Move: Use buffer stacking, 10–15% contingency on timeline + rehab budget + ARV cushion. Protect your downside on all fronts.

The Underwriting Safety Checklist

Before you go under contract, make sure your underwriting answers these critical questions:

ARV Reality Check

• Did you use only recently sold comps (not pending or active)?

• Did you adjust for finish level and size?

• Did you include softening trends in pricing if relevant?

Rehab Budget Check

• Did you walk the property (or get a video walkthrough)?

• Do you have a line-item rehab estimate (not just $/sq ft)?

• Did you build in a 15–20% contingency?

• Are permits, demo, and final cleanup included?

Timeline & Holding Cost Check

• Did you account for rehab duration and listing and close?

• Did you pull local DOM data?

• Did you calculate monthly burn rate (interest + fixed costs)?

• Did you stress test with 1–2 month delays?

If your answers aren’t solid, your underwriting isn’t either.

The Flip You Pass On Is the One You Profit From

You don’t make money by buying more deals; you make money by buying the right ones.

Underwriting is where you win or lose.

And these 3 mistakes?

• Fantasy ARVs

• Underestimated rehabs

• Timeline delusion

…they don’t just shrink profits. They kill them.

If you want to scale, survive, or even just stay sane as a flipper, your underwriting needs to be:

• Grounded in data

• Fueled by buffers

• Tested against time and reality

Run the Deal Through the Wringer, Or It’ll Wring You

Bad deals don’t start with bad drywall. They start with bad spreadsheets.

You don’t need to be conservative. You need to be brutal.

Because the flip game isn’t about optimism. It’s about downside protection and calculated upside.

Underwrite like your bank account depends on it, because it does.