BUSINESS COMBINATIONS (Acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended		6 Months Ended		12 Months Ended
	Oct. 28, 2019	Jul. 02, 2019	Mar. 08, 2019	Jul. 03, 2020	Jun. 28, 2019	Jul. 03, 2020	Jun. 28, 2019	Dec. 27, 2019	Jun. 26, 2019
Consideration for acquisitions
Cash paid							$ 21,800
Cash paid at closing								$ 27,000
Allocation of acquired assets
Goodwill				$ 130,236		$ 130,236		$ 127,647
Revenue and Income from operations
Revenues.				83,549	$ 104,396	189,575	196,189
Income from operations				(3,841)	2,773	(12,110)	2,539
Delayed Draw Term Loan Facility
Consideration for acquisitions
Maximum borrowing capacity									$ 50,000
E3, Inc.
Consideration for acquisitions
Tax deductible goodwill	$ 21,900
Issuance of common stock	$ 5,000
Number of trading days considered for calculation of weighted average price per share prior to closing date	10 days
Period over which financial targets must be met	3 years
Maximum Payout	$ 12,000
Proceeds from borrowings	30,000
Contingent consideration	7,000
Cash paid	25,217
Other working capital adjustment	1,973
Total consideration	39,190
Cash paid at closing	27,000
Allocation of acquired assets
Current assets	5,316
Non-current assets (1)	341
Cash	2,264
Equipment and leasehold improvements, net	409
Right-of-use asset	7,641
Current lease liability	(750)
Non-current lease liability	(7,300)
Liabilities	(4,325)
Goodwill	21,894
Net assets acquired	39,190
Increase in carrying value of right-of-used assets and non-current lease liability						1,500
Decrease in carrying value of liabilities and goodwill						1,500
Decrease in current assets				0		0
Revenue and Income from operations
Revenues.				6,800		11,800
Income from operations				1,600		2,400
E3, Inc. | Maximum
Consideration for acquisitions
Earn-Out Payments	12,000
Total consideration	44,000
E3, Inc. | Backlog
Allocation of acquired assets
Intangible assets	2,500
E3, Inc. | Customer relationships
Allocation of acquired assets
Intangible assets	8,300
E3, Inc. | Tradename
Allocation of acquired assets
Intangible assets	2,000
E3, Inc. | Non-compete agreements
Allocation of acquired assets
Intangible assets	$ 900
Onsite Energy Corporation
Consideration for acquisitions
Tax deductible goodwill		$ 8,600
Cash paid		24,905
Total consideration		24,905
Allocation of acquired assets
Current assets		19,058
Non-current assets (1)		10
Equipment and leasehold improvements, net		39
Right-of-use asset		828
Current lease liability		(168)
Non-current lease liability		(660)
Liabilities		(12,222)
Goodwill		8,600
Net assets acquired		24,905
Decrease in current assets						3,500
Increase in carrying value of goodwill						3,500
Additional amortization expense charge due to increase in fair value of intangible assets				700		700
Revenue and Income from operations
Revenues.				2,400		4,600
Onsite Energy Corporation | Backlog
Allocation of acquired assets
Intangible assets		1,510
Onsite Energy Corporation | Customer relationships
Allocation of acquired assets
Intangible assets		7,050
Onsite Energy Corporation | Tradename
Allocation of acquired assets
Intangible assets		860
The Weidt Group
Consideration for acquisitions
Tax deductible goodwill			$ 11,500
Cash paid			22,136
Total consideration			22,136
Allocation of acquired assets
Current assets			2,317
Non-current assets (1)			25
Equipment and leasehold improvements, net			198
Right-of-use asset			1,730
Current lease liability			(245)
Non-current lease liability			(1,533)
Liabilities			(612)
Goodwill			11,546
Net assets acquired			22,136
Revenue and Income from operations
Revenues.				3,700		7,400
Income from operations				600		800
The Weidt Group | Backlog
Allocation of acquired assets
Intangible assets			750
The Weidt Group | Customer relationships
Allocation of acquired assets
Intangible assets			4,240
The Weidt Group | Tradename
Allocation of acquired assets
Intangible assets			550
The Weidt Group | Developed technology
Allocation of acquired assets
Intangible assets			$ 3,170
Lime Energy
Unaudited pro forma financial information
Pro forma revenue				83,549	117,010	189,575	219,409
Pro forma income (loss) from operations				(3,841)	3,818	(12,110)	5,096
Pro forma net income (loss) (1)				$ (4,985)	$ 2,732	$ (13,139)	$ 2,784
Earnings (Loss) per share:
Basic (in dollars per share)				$ (0.43)	$ 0.24	$ (1.13)	$ 0.25
Diluted (in dollars per share)				$ (0.43)	$ 0.23	$ (1.13)	$ 0.23
Weighted average shares outstanding:
Basic (in shares)				11,682	11,315	11,593	11,252
Diluted (in shares)				11,682	11,894	11,593	11,885
Revenue and Income from operations
Revenues.				$ 12,900		$ 23,800
Income from operations				$ 2,200		$ 3,300
Willdan Energy Solutions | Onsite Energy Corporation | Maximum
Consideration for acquisitions
Total consideration		$ 26,400