Client accounting services was an explosive source of growth for the 2026 Top 100 Firms for the third straight year, with an overwhelming majority of reporting firms experiencing an uptick in the area.
It topped the charts of growing niches again this year, with 85% of the 88 responding Top 100 Firms experiencing a boost in this service line, up five percentage points from the previous year. Attest services also remained a reliable area of growth for the Top 100, again taking the No. 2 spot.
Estate/trust/gift tax planning, meanwhile, also retained its relevance — and positioning — this year in the No. 3 spot, while nonprofits and SALT tied as the No. 4 niches, with 70% of respondents seeing growth in these service lines. Rounding out the top five was M&A, a revenue booster for 69% of reporting firms.
On-demand and in demand
Myriad client needs and market shifts are accelerating the demand for client accounting services, and the 2026 Top 100 Firms are reaping the rewards.
Parsippany, N.J.-based Sax Advisory Group, for one, is turning industry challenges into CAS opportunities — backed by long-term strategic thinking, according to Peter Motsch, partner and leader of client accounting services. "At Sax, client accounting services has been our fastest-growing practice area," he shared. "Across the industry, a shrinking pool of accounting professionals and rising salary demands have made outsourcing the accounting function less of a luxury and more of a necessity."
"We see this differently," he continued. "CAS is not just a staffing solution — it's an opportunity. An opportunity for companies to move beyond reliance on a single accountant and instead become true partners with a firm that functions as their financial quarterback. Through that partnership, clients gain access to a team with collective knowledge across tax, advisory, wealth management and operations, supported by more than 75 partners and over 25 specialized areas of expertise — delivering depth, continuity, and strategic value no single hire can match."
(For more on strong niches and client service areas among the Top 100,
New York City-based Grassi is experiencing a similar ROI, reports Louis Grassi.
"As businesses increasingly seek flexible, outsourced solutions, our continued investment in client accounting services reflects both market demand and our commitment to delivering support that scales with our clients' needs," he explained. "Beyond traditional accounting and bookkeeping, our CAS practice has evolved into a strategic offering that includes cash flow forecasting, budgeting, and advanced capabilities that enhance visibility, control and execution across the business."
Birmingham, Alabama-based Warren Averett's CAS practice similarly includes a wide range of services.
"We've seen an increased demand for client accounting services as more business owners pursue real‑time financial insight and want advisors who can help them manage risk, improve efficiencies, and identify growth opportunities," said Amber Stout, client accounting services leader. "Our CAS team is successfully helping clients navigate uncertain economic conditions by delivering budgeting, forecasting and strategic financial guidance."
The professionals in Indianapolis-based Katz, Sapper & Miller's CAS practice also serve as guides for clients and their businesses.
"Our growth is driven by strong demand from business owners and leadership teams who want accurate financials, but more importantly, clarity to make better decisions around growth, cash flow, and resource allocation," explained Zach Sauder, partner and the leader of the outsourced finance and accounting services group. "Our team is built on a mix of private and public industry experience, including CFOs, controllers and business owners, which allows us to connect with clients as operators, not just accountants. That perspective helps us translate financial data into actionable insight tied directly to the decisions leaders are facing. Clients value that we deliver exactly the level of support they need, no more and no less, while remaining flexible as those needs evolve."
Tampa, Florida-based Crete Professionals Alliance, with its network of more than 20 firms, offers an even broader perspective on the growth and success of the CAS niche.
"Across our firms, including RRBB, AbitOs, Abacus, Bowers, Reid, and Adeptus, client accounting and advisory services is growing as business owners look for more than compliance," shared Crete CEO Steve Stagner. "They want real-time insight, forward-looking guidance, and help navigating multistate and international complexity."
As with all high-growth areas, change management is a common challenge for firms with CAS practices. "The primary challenge has been scaling thoughtfully while preserving the high-touch, strategically focused nature of our work," said KSM's Sauder. "As demand increases, it is critical to maintain consistency, quality, and alignment with our advisory-first philosophy. We are intentional about building teams with the right experience and mindset."
Crete reports similar obstacles. "The challenge has been scaling talent, systems and infrastructure fast enough to meet demand while maintaining consistency and margins," said Stagner. "Crete is addressing this by standardizing CAS delivery models across firms, investing in automation and AI-enabled platforms to streamline routine work, and building shared processes that free professionals to focus on higher-value advisory and consultative services."
Many of the Top 100 Firms' CAS practices are successful for the kind of strategic planning that practice leaders expect to continue driving growth into the future.
"Looking ahead, we expect CAS demand to keep accelerating," predicted Sax's Motsch. "However, the firms that win long term will be the ones with integrated capabilities under one roof, investing in technology and team depth, not just absorbing overflow work. The goal isn't to replace a client's accountant; it's to give them something better than any single hire could provide."
CAS is also a safe bet going forward for Crete, according to Stagner, who said the firm's investment in the niche "is driving predictable recurring revenue, stronger margins, and deeper advisory relationships with both new and existing clients. We expect CAS to remain one of our strongest growth engines as we continue scaling talent, technology, and operating leverage across the platform."
The outlook is also bright at KSM. "We expect continued growth as more organizations seek finance partners who understand their business, not just their books," said Sauder. "Our focus will remain on providing right-sized, high-impact support that scales with clients and keeps strategy at the center of the relationship. As competition intensifies and technology and AI accelerate the pace of change, the bar for financial insight continues to rise. Businesses increasingly need advisory-led teams with real operating experience that can help leaders interpret data quickly and make confident, forward-looking decisions. That positions us well for sustained, disciplined growth."
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The No. 2 high-growth niche, attest, was an intentional area of focus for Princeton, N.J.-based Withum.
"Our attest growth reflects a strategic decision to lean into the SEC practice at a time when many firms have pulled back due to regulatory pressures," said audit partner Ed O'Connell. "By investing in technical resources, audit quality, and talent development, Withum has been able to grow responsibly while meeting the demands of an increasingly complex regulatory environment. Going forward we anticipate continued growth as we continue to invest in our team and infrastructure to meet rising expectations."
For Canfield, Ohio-based Hill, Barth & King, growth in the firm's estate/trust/gift tax planning services was largely driven by the market and regulatory change.
"We're in the early stages of the largest intergenerational wealth transfer in history, driven by baby boomers who hold more than half of all U.S. household wealth reaching retirement age," reported Amy Dalen, principal and national tax director. "We are also seeing an increased focus on privacy, asset protection, and wealth preservation. Recent changes to the federal estate and gift tax exemption, and continued state estate tax and inheritance tax complexity, are driving thoughtful, comprehensive planning that touches on multiple areas of the Tax Code and requires coordinated tax strategies. Our clients are eager to engage in these planning discussions, and the demand for comprehensive wealth preservation and transfer expertise at CPA firms will only continue to grow."
